An odd little agreement
Agreement can, it seems, occur in the most unexpected of places. Though few if any Protestant theologians of the Reformation and Post-Reformation eras would admit to employing or even agreeing with Thomas Aquinas, indeed in many circumstances such an admission would have lead to immediate accusations of heresy, there are times when the brilliance of the Angelic Doctor is such that his ideas are mirrored by those who would outright reject him.
One such little agreement is between Thomas’ sacramentology and that of Philipp Melancthon, the sadly-overlooked leader of early Lutheran theology and arguably co-founder of Lutheranism along with the eponymous Martin Luther. In the Apology of the Augsburg Confession, which now enjoys a place within the Book of Concord, Melancthon, in his explanation of sacrifice and the use of the sacrament of the Eucharist, makes the following comment:
Sacraments are signs of God’s will toward us, not simply sign of the people’s will among themselves, and so it is right to define the New Testament sacraments as signs of grace. (XXIX, 69)
This position, as any good edition of the Book of Concord will point out, mirrors the sacramental thinking of Thomas Aquinas’ Summa Theologiae:
The principal cause cannot properly be called a sign of its effect, even though the latter be hidden and the cause itself sensible and manifest. But an instrumental cause, if manifest, can be called a sign of a hidden cause, for this reason, that it is not merely a cause but also in a measure an effect in so far as it is moved by the principal agent. And in this sense the sacraments of the New Law are both cause and signs. Hence, too, is it that, to use the common expression, they effect what they signify. From this it is clear that they perfectly fulfill the conditions of a sacrament; being ordained to something sacred, not only as a sign, but also as a cause. (III, q.62, a.1, ad 1)
It seems fairly unlike that Melancthon would be employing the work in the newly-prestigious Thomas Aquinas in his thought. Though at the time Thomas was far from the normative or preeminent Catholic theologian that he would become, the work of Cardinal Cajetan and the birth of the Thomistic Renaissance being only in its very earliest days when Melancthon was writing, his earlier commentary on the Sentences of Peter Lombard was widely in use and he was a respected theologian in Catholic circles. More importantly perhaps he was a scholastic theologian, and the Reformers were so ardent in their rejection of what they understood to be scholastic sophistry and its obscuring of ‘plain’ biblical truth. However the similarities between these two theologies are undeniable, and in this case correlation may not suggest causation so much as coincidental agreement.
The question is raised as to whether Melancthon may, in fact, have been influenced by Thomas or his scholastic contemporaries. Melancthon is often anachronistically described as a ‘systematic’ theologian  and his work could certainly be said to bear as much, if not more, resemblance to that of his own scholastic contemporaries as Humanistically inclined scholars, Protestant or Catholic. Of course, one could also see him as simply the first in the ‘systematic’ tradition that would give rise to Protestant Scholasticism. It is certainly a question deserving of further study.
Notes and bibliography
Muller, Richard A., God, Creation, and Providence in the Thought of Jacob Arminius: Sources and Directions of Scholastic Protestantism in the Era of Early Modern Orthodoxy (Grand Rapids: Baker House Books, 1991), 37.
Robert Kolb and Timothy Wengert, eds., The Book of Concord: The Confessions of the Evangelical Lutheran Churches (Minneapolis: Fortress Press, 2000), 270.
Fathers of the English Dominicans, trans., St. Thomas Aquinas Summa Theologica: Complete English Edition in Five Volumes (New York: Benziger Bros., 1948).
Muller, God, Creation, and Providence, 35.
See, for example; his Wikipedia entry.