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The best overview of the Mercersburg movement is probably the horribly-named Romanticism in American Theology. The only  problem here is that Nichols seems to project his own views of the  reliability and authority of Scripture upon Nevin–at least he doesn’t  provide much evidence for what he claims is Nevin’s view. A better  source would be from “The Library of Protestant Thought” in which one  volume is The Mercersburg Theology (New York: Oxford University  Press, 1966), which is edited by Hastings. By far the best way to  acquaint oneself with them is to read the books by Nevin and Schaff for  oneself. The following introduction relies primarily on the introductory  material in Jack Martin Maxwell’s Worship and Reformed Theology: The Liturgical Lessons of Mercersburg (Pittsburg: The Pickwick Press, 1976), which contains a concise summary  of the history of Mercersburg in it’s introductory material.  (Henceforth: WRT).
The “Mercersburg Movement” was principally begun and propagated by  John Williamson Nevin and Philip Schaff, professors at the German  Reformed Mercersburg Seminary. After years as a Presbyterian quite  committed to the teaching he had received at Princeton, Nevin began a  shift in theology which involved a new understanding of the historic  development of doctrine and a corresponding vision of the Church as a  growing entity, a respect for the ancient and medieval Church, a more  thoroughgoing awareness of and loyalty to the sacramental theology of  the sixteenth-century reformers. This shift seems to have started with  his exposure to the tracts of the Oxford movement and German philosophy  and theology, fueled by an opposition to “new measures” revivalism which  he apparently (and ironically) picked up from Charles Hodge.[WRT, pp. 11-15]  When Philip Schaff left his homeland in Germany and joined Nevin, the  Church historian found a person with whom he shared a common vision.
The Mercersburg Theology stressed the centrality of the incarnation.  As we will see, Nevin insisted that the atonement was necessary for  salvation, but he violently rejected the idea that the incarnation was  simply for the purpose of the atonement, which would render it as simply  a means to an end. Rather, the incarnation was an end in itself,  whether or not sin necessitated the atoning death and justifying  resurrection of Christ. Through union with Christ, man can have union  with God [WRT, pp 25-26. However, Maxwell writes: “Nevin contended that as man is  in and identified with Adam’s guilt, so he is in and identified with  Christ’s perfect life; and this latter identification results in a  “mystical union.” This reverses Nevin’s thought. It is because the Holy  Spirit gives one “mystical union” with Christ, that one “is in and  identified with Christ’s perfect life.”].
The Church is the continuation of Christ’s life on earth through the  agency of the Holy Spirit. The Church is not simply a collection of  believers, but the mystical body of Christ, the mother of all believers.  Nevin discarded the categories of “visible” and “invisible,” discussing  instead the “actual” Church (present) and the “ideal” Church  (eschatological). The ideal exists as a seed in the actual, and  inexorably takes shape in history until the resurrection [WRT, pp 29-30].
The mystical union between Christ and His Church made the sacraments  quite important in the Mercersburg view. Nevin promoted a return to  Calvin’s doctrine of the Lord’s Supper as the means by which the  Church’s union with Christ is nourished and strengthened. In defending  this view, he touched on almost all the distinctives of the Mercersburg  Theology.
Nevin never set out to specifically discuss the imputation of Adam’s sin. (This is probably the main reason no one ever thought to evaluate his  views in the context of nineteenth-century Presbyterian controversy.  Those who study Nevin are not the sort of people prone to care enough  about Reformed orthodoxy to keep track of the other doctrinal debates  between Old Princeton, the Southern Presbyterians, etc.)  Rather the imputation of Adam’s sin is mentioned as an aside to his  discussion of the imputation of Christ’s righteousness, which in turn  was only mentioned by Nevin to explain why the Reformation Tradition  found it so important to affirm our union with Christ and the importance  of Christ’s real presence in the Lord’s Supper. The Mystical Presence: A Vindication of the Reformed or Calvinistic Doctrine of the Holy Eucharist sets forth Nevin’s views on the sacrament, and covers all these (for his purpose in the book) subsidiary issues. Not only is Mystical Presence Nevin’s most thorough treatment (and one that articulated views which  remained essentially unchanged for the rest of his life), but it was  also the presentation to which Hodge responded.
Thus, the best way to explain Nevin’s view of the imputation of  Adam’s sin is to first briefly set forth from this work his beliefs  concerning the importance of union with Christ and the imputation of his  righteousness, as well as the nature of this union.
The Importance of Union with Christ
Nevin would emphatically agree with the Liberal catchphrase, “Christianity is a life, not a doctrine.”[See The Mystical Presence and Other Writings on the Eucharist,  vol 4 of Lancaster Series on the Mercersburg Theology, Bard Thompson  & George H. Bricker, ed. (Philadelphia, Boston: United Church Press,  1966), p. 216ff. Henceforth: MP].  Yet just as emphatically, he would insist that only by proclaiming  Christianity as a life and not a doctrine can supernatural Christianity  be set apart from rationalistic naturalism. Socinians, by reducing  Christianity to a moral message, throw “the man back always upon  himself, his own separate powers and resources, the capabilities of the flesh as such, to perfect his nature and make himself meet for heaven.”[MP, p. 186] Likewise, in Pelagianism, “we are thrown back again, upon such material in the way of life,  as the subject of it may be found to possess in his own nature, when  brought under the action of this divine process of education” [MP p. 187].
So far, this is rather standard fare. But Nevin takes a further  ingenious step: What about those who claim that salvation depends on the  supernatural enlightenment of the Holy Spirit? Of such a view, he says:
To the force that belongs to the truth itself in its relation to the human mind, it may   join the influence’s of God’s Spirit,  graciously interposed to clothe the truth   with effect. Such agency we  often hear attributed to the Spirit, by those who at the same   time  reject altogether the thought of any immediate change wrought  by it in the   nature of the human soul itself. God’s grace in this  form, they say, is brought to bear on   the soul, mediately only, by the intervention of his word which he uses   instrumentally for  the purpose, infusing into it light and power. But surely those who    talk in this way do not stop at all to consider the exact sense of their  own words. What   do they mean, when they speak of the Spirit, as  infusing light and power into the truth?   Can he do so (apart from a  direct influence on the soul itself) in any other way than by   so  ordering the presentation of the truth to the mind, that it shall be  placed in the most   favorable position for exerting the power which  belongs to it in its own nature? But what   is this more than such moral  suasion, as may be exercised over the spirits of men in a   merely  human way, by appeals addressed to the understanding and will? The order  of   influence at least remains the same, though it may be exhibited  under a divinely exalted   form [MP, pp. 187-188].
This view, though partially supernaturalistic, still falls back into  naturalism on the crucial issue of salvation, and still does not escape  the error of Socinianism and Pelagianism:
In this view, the process of salvation, in the midst of  all the high-sounding terms   that may be employed to describe it, falls  back again to the standpoint already noticed.   It is a salvation by  the power simply of truth, presented in the form of doctrine and    precept. This truth includes the supernatural facts of the gospel, the  mission,  sufferings, death, and resurrection of Christ–the outward  apparatus in full, if we may use the expression–of the Christian  redemption; and along with this we have the “moral   suasion” of the  Holy Spirit, which according to the unintelligible hypothesis,   invests  the whole representation with a more than natural evidence and power.  All turns at   last, however, on the way in which the mind thus  addressed, may be wrought upon and moved   to act, in the use of such  resources and capabilities as are already comprehended in its   nature [MP, p. 188. Though it is outside the bounds of this paper, it would be  interesting to investigate to whom Nevin was particularly responding. I  can’t help noticing a similarity between the view Nevin repudiates and  the view of regeneration ascribed to Hodge by Dabney. According to  Dabney, Hodge reduced the work of the Spirit in regeneration to  enlightenment of the mind, and was virtually guilty of Pajonism. See  “Hodge’s Systematic Theology” in Discussions: Evangelical And Theological vol 1 (London: Banner of Truth, 1890, 1967), pp. 231-253)].
Even affirming regeneration through the power of the Holy Spirit is  not enough to escape the problem. It remains if the union with the  Spirit does not also involve intimate mystical union with Christ’s new  humanity, if “Christ dwells in his people by his Spirit–but in the way  only of representation, not in the way of strict personal inbeing on his  own part” [MP, p. 195].
The same Spirit, it is said, that works in Christ works  also in us, fashioning us as we   are into the same image. But how does  he work? By supernatural influence, it may   be said. But is  not this to fall back again to the theory of a merely moral union with    Christ, by the power of the truth only; which we have found already to  be under its   highest form, but Pelagianism in disguise? Is Christ in  us at last only by the divine suasion of his Spirit? [MP, pp. 197-198].
Nevin goes on to consider the only supernaturalistic  alternative which avoids mystical union: “The Spirit, it may be said,  creates new life in the believer.” Yet this involves insuperable  difficulties: “But what now is this new life? Something, of course, that  was not in the man before. From where, then, does it come? Is it the  proper life of the Spirit himself–the life of God–directly extended to  the soul? This would be to repeat the mystery of the incarnation, in the  case of every new believer… From where, then, we ask again, comes this  new life by the Spirit? Is it an absolute creation out of nothing…?  Instead of one great miracle, then, in Christianity–the new creation in  Christ Jesus–we should have miracles of the same order without number or  end. Every believer would be a new creation, not in Christ Jesus, but  in himself…” [Ibid].
This conception of regeneration, then, makes it not an ingrafting  into Christ, but some sort of merely moral transformation, as if man  could be saved through some sort of change in his own fallen condition.  Because man is totally depraved and has fallen irrevocably in Adam,  there is no miracle which can correct the problem of man’s sin and  guilt–except to send God as a new man, the second Adam, to provide a new  source of life to conquer the death spread from the old man, and then  give that life to men dead in their sins.
The Imputation of Christ’s Righteousness
Nevin has not yet exhausted the attempted alternatives to the  mystical union with Christ. American protestants, realizing that the  suasion of the Spirit is not enough to give them a truly supernatural  soteriology, think they have yet another option:
Here we are brought, then, to stand upon higher and more orthodox ground. The doctrine   of imputation is introduced, to meet the demand now mentioned. The work of Christ    is no longer thought of as a mere display for moral effect; it is  something to be   appropriated and made available in the person of the  believing sinner himself, for the   purposes of salvation. Mere doctrine  will not answer. The case calls for an actual   personal participation  in what Christ has done and suffered to take away sin and reconcile    man to God. By imputation. we are told. As the guilt and fall of Adam  were reckoned to his   posterity, though not theirs in fact, so the  righteousness of Christ, and the benefits of   his mediatorial work  generally are, in virtue of the terms of the new covenant, made over    to all who believe in his name, and accounted to be theirs as truly as  though all had been   wrought out by them, each for himself, in truth.  Their justification in this view is a   mere forensic act on the part of  God, which is based altogether on the work of Christ, and   involves as  such in their case no change of character whatever, but only a change  of   state. God regards them as righteous, though they are not so in  fact, and makes over to   them a full title to all the blessings  comprehended in Christ’s life. At the same time, he   regenerates them  by his Spirit, and puts them thus on a process of sanctification, by    which in the end they become fully transformed in their own persons,  into the image of   their glorious Savior [MP, pp. 188-18].
This imputation appears to escape the problem of naturalism in  Nevin’s mind. Nevertheless, it is an insufficient explanation because it  is flatly impossible. “The imagination that the merits of  Christ’s life may be sundered from his life itself, and conveyed over to  his people under this abstract form, on the ground of a merely outward  legal constitution, is unscriptural and contrary to all reason at the  same time” [MP, 192].
The judgment of God must ever be according to truth. He  cannot reckon to anyone an   attribute or quality that does not belong  to him in fact. He cannot declare him to be in a   relation or state  that is not actually his own, but the position merely of another. A    simply external imputation here, the pleasure and purpose of God to  place to the account   of one what has been done by another, will not  answer. Nor is the case helped in the least   by the hypothesis of what  is called a legal federal union between the parties, in   the  case of whom such a transfer is supposed to be made; so long as the law  is thought of   in the same outward way, as a mere arbitrary arrangement  or constitution for the   accomplishment of the end in question. The  law in this view would be itself a fiction   only, and not the  expression of a fact. But no such fiction, whether under the name of law    or without it, can lie at the ground of a judgment entertained or  pronounced by God [MP, pp. 190-191].
In explaining why a “bare” legal imputation is not enough, Nevin knew  that the accusation would be made that he was denying justification by  Faith. He (futilely) attempts to cut off this line of attack: “Do we  then discard the doctrine of imputation, as maintained by the orthodox  theology in opposition to the vain talk of the Pelagians? By no means!  We seek only to establish the doctrine; for without it; most assuredly, the whole structure of Christianity must give way” [MP, 190].
Christ’s righteousness is imputed to us, writes Nevin, when the Holy  Spirit actually gives us union with Christ. If we have union with Christ  we possess all that is His. His active and passive righteousness count  for us because “He is joined to us mystically” [MP, p. 192].  Christ’s righteousness is truly imputed, and justification is truly  forensic and declarative, but the basis is not simply God’s imagination  that we are justified, but “our actual insertion into Christ himself” [MP, p. 192].
The Nature of the Union
How are we united to Christ and to Adam? What does Nevin mean by  “mystical union”? In what sense is the incarnation so all important to  this union, so that this union, though with His whole Person, especially  involves his humanity? This question becomes more acute when we realize  that Nevin is insisting on following Calvin that in the Eucharist we  partake of Christ’s flesh and blood without any transfer of particles or  physical presence involved! All this relates to the point of this  study: How is this union with Christ parallel to our union with Adam?  And how does this union with Adam undergird the imputation of Adam’s sin  to his posterity?
Nevin explains himself by making the rather bold claim that a living  organism is not reducible to material particles. He uses the  relationship of an acorn to an oak tree to prove this idea [MP, p. 156]. We classify an acorn and the tree which grows from it as a single organism–the seed becomes the tree. Yet, the tree is exponentially more massive than the acorn,  and has obviously acquired mass from the soil around it. Indeed, it is  easily possible that the old oak tree does not contain a single material  particle which was present in the acorn. Yet the lack of identical  material particles means nothing. The life of the acorn is the same life  which animates the leaves on the tree. The branches are connected to  the root by a shared life which cannot be reduced to material particles.  Furthermore, in thinking this way, it becomes apparent that to limit  the life of the acorn to the single oak tree is quite arbitrary–for the  oak bears acorns from its life which grow themselves into other trees.  “Still, in the end, the life of the forest, in such a case, is nothing  more than an expansion of the life that lay involved at first in the  original acorn” [MP, p. 156].
Thus, the life of Christ’s flesh and blood is not found in any  physical particles, but in an animating force or “law.” A “clear  distinction” must be made between
the idea of the organic law, which constitutes  the proper identity of a human   body, and the material volume it is  found to embrace as exhibited to the senses. A true   and perfect body  must indeed appear in the form of organized matter. As a mere law, it can have no proper reality. But still the matter, apart from  the law, is in no   sense the body. Only as it is found to be transfused  with the active presence of the law   at every point and in this way  filled with the form of life, can it be said to have any   such  character. . . The principle of the body as a system of life, the  original salient   point of its being as a whole, is in no respect  material. It is not bound of course, for   its identity, to any  particular portion of matter as such. If the matter which enters into    its constitution were changed every hour, it would still remain the same  body. . . A real   communication then, between the body of Christ and  the bodies of his saints, does not   imply necessarily the gross  imagination of any transition of his flesh as such into their    persons [MP, p. 151].
Thus, by the mediation of the Holy Spirit, we can truly and really  participate in the life of Christ. We can be united with His flesh and  blood. This is an ultimately mysterious identity, yet it is the same  sort of mystery which confronts us in all living beings.
The Imputation of Adam’s Sin
At this point we can easily see how Nevin understands the unity of  the human race with Adam. Nevin is quite certain that, just as a “mere  outward imputation” would be impossible in the case of Christ’s  righteousness, so would it be in the case of Adam’s sin.
Can we conceive of any constitution, for instance, in  virtue of which it could have   been proper or possible for the Divine  Mind, thus to set over to the account of mankind   the apostasy of  angels, which kept not their first estate, the two natures being    relatively to each other what they are at this time? If all depended on  the arbitrary   pleasure of God, the force of a mere outward arrangement  constituting one the   representative of another without further  relation, we cannot see why the transfer of   guilt might not take place  from angels to men, as well as from Adam to his posterity. The   very  fact that our whole reason and feeling revolt against the thought of the  first case,   serves only to show that the proceeding must rest upon  some deeper ground in the   other [MP, p. 191].
In the case of Adam’s sin, the analogy of the acorn and the oak tree  can be more literally applied. We are all Adamites. Our bones are  Adamite bones; our flesh Adamite flesh; and our very life a true  continuation of Adam’s life. The fact that the billions of individual  human beings are made up of material particles other than those which  originally constituted Adam when he was first created is utterly  irrelevant. For all we know Adam himself, at the time of his death, may  have been constituted by a completely different set of particles from  those which constituted him 920 years earlier. The fact is that we all  grew out of him and are no less a part of him, in one sense, than a  branch is part of a tree.
It is interesting that Louis Berkhof wrote in objection to “the  realistic theory” that: “Every man is conscious of being a separate  personality, and therefore far more than a mere passing wave in the  general ocean of existence” (Systematic Theology [Grand Rapids,  MI: Eerdmans, 1941], p. 241). I doubt that Nevin was a realist, but he  would probably respond that every man is a separate personality and a  “wave” of a sort. Indeed, Nevin’s “law” sounds quite similar to the  “wave form” which provides the backdrop to Tim Powers’ science fiction  ghost story, Expiration Date (New York: TOR, 1996):
And he  remembered the old notion that after some number of years every cell in a  human body had been replaced, every atom, so that the body is just a  wave form moving through time, incorporating just for a little while the  stuff of each day; only the wave itself, and none of the transient  physical bits, makes the whole trip. Even a scar would be no more  significant than a wobble still visible in an ocean wave long after the  wave had passed the obstruction that caused it, while the water  molecules that had actually sustained the impact were left comfortably  behind” [p. 147].
By his fall, Adam became corrupted in his nature, and all his  children who come from his nature share in that corruption. What he did  freely, Adam’s children continue to do spontaneously and naturally. They  inherit his sin and his guilt.
Just as he expected some to accuse him of denying justification by  Faith, Nevin knew others would accuse him of denying the Reformed  doctrine of the imputation of Adam’s sin. Thus, he took the space to  argue that the Westminster Standards are not guilty of reducing original  sin to a “mere outward imputation.” On the contrary, “The language of  the catechism is literally and strictly correct. We sinned in Adam, and fell with him, in his first transgression.” Furthermore, question eighteen of the  Shorter Catechism does not define original sin as only “the guilt of  Adam’s first sin,” but lists a threefold definition which also includes  “the want of original righteousness” and “the corruption of his whole  nature.”
Nevin admits that “the friends of the catechism, in their attempts to  vindicate its doctrine at this point, have not always planted  themselves on the proper ground for its defense,” because they have  rested their case on “a merely external imputation” which can give us  “only a quasi interest in the real fact that it represented” at best.  But in so doing they are not only failing to defend the doctrine, but  inadequately stating what the catechism actually claims [MP, p. 191].
TO BE CONTINUED