Shamelessly stealing Aaron's "Calvin contra . . ." title form while trying to slip into Mark's stream of interest in merit, I thought this might be a good time to indulge a little excursion in Reformed diversity by noting Calvin's peculiar position on the source of Christ's merit.
At least since Anselm the merit of Christ's satisfaction on our behalf has been linked to the infinite worth of the divine person accomplishing that work. As the eleventh century closed, the good Archbishop of Canterbury, while in exile, famously asked Why the God-man (Cur Deus Homo)? To answer, he argued that only God could satisfy what humanity owed, and strongly suggested a line of argument later elaborated by Thomas Aquinas (among others), who grounded the merit of Christ in the person of the Son. The idea is probably familiar to most readers: the great worth of Christ's work comes from the fact that he is the infinitely worthy Son of God who freely assumed a fully human nature in order to accomplish our redemption.
Obviously, significant differences exist between Medieval and later Reformed thinkers on the merit of Christ. Yet, on the source of Christ's merit, the bulk of the Reformed tradition, it seems, follows Thomas. Richard Muller makes this very point in his entry on meritum Christi in his Dictionary:
The argument most often found among the Protestant scholastics, both Lutheran and Reformed, received its clearest medieval formulation in the thought of Thomas Aquinas. The source of the meritum Christi is the persona Christi who performs the work of satisfaction. . . . Since the person is the divine Word, the infinite Second Person of the Trinity, the work performed by that person, even though accomplished through the instrumentality of his human nature, must be infinite.
This is exactly the argument we find in that titan of continental Reformed scholastics, Francis Turretin. Maintaining that the perfection of Christ's satisfaction excludes any possibility of a human contribution in this life or need for purgatory in the next (contra "the Romanists"), he contends that "the perfection of this satisfaction" is due,
First, [to] the dignity of the person satisfying; this was not only holy and most pure, but also truly divine. . . . We cannot doubt that this satisfaction which he has made is one of infinite value and efficacy and therefore of such fulness and all-sufficiency that nothing can be added to it. For although his human nature (which was the instrument in the obedience and sufferings) was finite, yet the satisfaction itself does not cease to be infinite (relative to the person, which is here the efficient cause and to which they ought to be attributed) (Elenctic Theology, Q14.12.7).
Here, Turretin follows Thomas, reasoning that the infinite merit of Christ's satisfaction is due to the divine person who rendered that satisfaction on our behalf.
Calvin, however, develops a different line--one that makes no appeal to the infinite worth of the divine person but looks instead to the decretive will of God. We find this in his discussion of how we can correctly say that Christ has merited grace and salvation for us. Here he argues that,
When we treat of the merit of Christ, we do not place the beginning in him, but we ascend to the ordination of God as the primary cause, because of his mere good pleasure he appointed a Mediator to purchase salvation for us (Institutes, 2.17.1).
Instead of an appeal to the divine person of the Word incarnate who was that mediator, Calvin appeals to the arrangement decreed by God out "of his mere good pleasure." The argument is important to Calvin, in context, because it demonstrates, he believes, that the merit of Christ is not opposed to the mercy of God. On the contrary, it was God's mercy to appoint a mediator and arrange the state of affairs by which that mediator could merit salvation for us. In this way, he argues, the merit of Christ is always "in subordination to" the "mere mercy of God" (2.17.1).
Calvin goes so far as to argue that "Christ could not merit anything save by the good pleasure of God," meaning that "the merit of Christ depends entirely on the grace of God (which provided this mode of salvation for us)" (2.17.1).
Curiously, Calvin's argument has a decidedly Scotist ring to it. In typical Duns-ian fashion, the Scot developed a voluntaristic alternative to Thomas's appeal to the divine person, arguing that since Christ's work was accomplished by the Son as a man it necessarily has a finite value. As such, the sufficiency of Christ's work--its infinite merit--is grounded in God's counterfactual acceptance of his work as a full satisfaction for sin.
That, to be clear, is not Calvin's argument. Although both Scotus and Calvin agree that the will of God is the source of Christ's merit, Calvin argues that Christ's work has infinite merit on the basis of God's decree. The difference may seem subtle but is significant: Scotus's argument from the divine will to accept Christ's work as counterfactually sufficient is later developed by Hugo Grotius into his moral governmental theory of the atonement. Calvin's view precludes such development just as completely as Thomas's before him or Turretin's after him, but on a surprisingly different premise.