Zacharias Ursinus was the principal author of the Heidelberg Catechism and wrote (or allowed to be put together from student notes on his lectures) a commentary on the same. It is available online here.

He wrote that, among other ways, law and gospel differed

In the promises which they make to man. The law promises life upon condition of perfect obedience; the gospel, on the condition of faith in Christ and the commencement of new obedience (p. 3).

Notably, Ursinus is not talking about the Mosaic Covenant when he speaks of “law” here. Rather, the Mosaic Covenant and the Gospel Covenant are the same in substance because they make the same promises on the same conditions.

There is but one covenant, because the principal conditions, which are called the substance of the covenant, are the same before and since the incarnation of Christ; for in each testament God promises to those that repent and believe, the remission of sin; whilst men bind themselves, on the other hand, to exercise faith in God, and to repent of their sins (p. 99).

The “law” then refers to the perfect obedience that Adam was supposed to persevere in as a condition for inheriting glory. The Mosaic Covenant, was for Ursinus, just as it was for the Westminster Divines, an administration of the one covenant of grace.

Furthermore, Ursinus did not believe that any such perfect obedience, if it had been rendered, could have been meritorious. When he explains why our good works are necessary as conditions but not meritorious he gives reasons both why 1. a sinner’s works could never be meritorious, 2. why a creature’s works could never be meritorious. Of the latter he writes.

No creature, performing even the best works, can merit any thing at the hand of God, or bind him to give any thing as though it were due from him according to the order of divine justice…(Romans 11.35; Matthew 20.15). We deserve our preservation no more than we did our creation. God was not bount to create us; nor is he bound to preserve those whom he has created. But he did, and does, both of his own free-will and good bleasure. God receives no benefit from us, nor can we confer anything upon our Creator. Now, where there is no benefit, there is no merit; for merit presupposes some benefit received.

Our works are all due unto God; for all creatures are bound to render worship and gratitude to the Creator, so that if we were even never to sin, yet we could not render unto God [more than?] the worship and gratitude which is due from us… (Luke 17.10). (p. 486)

As you can see, I’m not comfortable with the last sentence quoted above since it seems to me to claim that a perfect creature could not do what was due. I think Adam had the ability to do what was required but decided to repudiate God.

In any case, the above considerations apply to creatures, not merely sinners. Notably, when Ursinus speaks of merit, he ascribes it only to the Incarnate Son of God who was not a mere creature. I need to study this more but I get the impression he also restricts it to Christ’s sacrifice that was designed to satisfy God’s justice on account of Adam’s demerit.

For example:

All these things Christ does, obtains, and perfects, not only by his merits, but also by his efficacy. He is therefore, said to be a Mediator, both in merit and efficacy; because he does not only by his sacrifice merit for us; but he also, by virtue of his Spirit, effectually confers upon us his benefits, which consist in righteousness and eternal life… (p. 94)

The office of the mediator consists of two parts: his humiliation or merit; and his glorification or efficacy. Now as it respects his humiliation, Christ is meritorious; as it respects his glorification, he is efficacious (p. 164).

It is by making intercession for us in this manner that Christ applies unto us the benefits and merit of his death (p. 250).

So, to sum up.

1. The contrast between law and gospel is the contrast between the requirement of perfect obedience before the Fall and the requirement of faith and the commencement of new obedience or repentance after the Fall.
2. After the Fall, the substance of the covenant of Grace remains the same before and after Christ because the conditions involved in the promises or the same.
3. Before the Fall, the demand for perfect obedience was not a demand for merit because it is impossible for creatures to merit eternal glory from God.
4. For Christ, the issue of merit seems to be (at first glance) related exclusively to Christ’s sufferings or humiliations that are aimed and satisfying God’s justice for the offense of Adam’s and then our own demerit.

I won’t have time for awhile, but I would like to study point 4 above so I can either disprove it or speak with more confidence about it.

See also: Zacharias Ursinus and the Imputation of the Active Obedience of Christ