Now the reason why God authorized men to use a rite of this nature, involving immersion or washing or sprinkling with water and received at the hands of the official ministers of religion, as the means of obtaining the washing away of their sins and hence also as the regular mode of initiation to the service of God, is to be found in his purpose to confirm and stimulate to greater vigor in them by this procedure the first and foremost principle of our salvation, namely, faith in the remission of sins, that is, in our unmerited justification. For God himself formed us in such a way that whenever we are seriously promising or conferring invisible realities our natural inclination is to do so by means of signs perceptible to the senses. The same procedure can be observed among all peoples in important transactions of every kind, for it is in this manner that treaties are concluded, kings installed, marriages contracted, and sales executed.
Consequently, as far as this use of symbols is concerned, God deals with us in terms of our own practice, as he is accustomed to do in other respects as well. And since the whole of the covenant he has made with us and our entire salvation (which is his primary consideration in all his dealings with us) have their beginning and basis in our persuasion that he pardons our sins, in his wisdom he has willed to confirm and stimulate our faith in this pardon principally by his own symbol, and particularly at the time when men consecrate themselves to his service in a special way. For on that occasion they reflect more closely on their own unworthiness and his goodness, and as a result more fully forsake self and dedicate themselves to him for a life of complete holiness and a true readiness to serve the needs of all men.
His purpose, however, to present the remission of sins through the agency of public ministers of religion was not determined solely by the fact that it is appropriate for physical symbols to be conferred at the hands of men. It was also his aim by this means to knit his own more closely together and to each other, and to bind them more securely to submission to religious instruction and admonition in the congregation. This should result from their realizing that the men from whom they received the counsels of salvation and to whom they must cleave as fellow members in the same body are able to shut or to open heaven to them, and to retain or to remit their sins. The Church of God, of course, has always possessed this power, and God has never failed to make use of its exercise for the salvation of his own whenever the Church has languished in spirit and the light of knowledge.
It should now be clear from what we have said why God has required his Church in every age to use baptism and in this manner to introduce men to his service.
Source: Martin Bucer, “Baptism,” Martin Bucer, Courtenay Reformation Classics IV, pp. 287, 288.