Over at Steven Wedgeworth's Wedgewords, Peter Escalante has, in the context of a discussion about civil authority, given a fairly succinct overview of one of the key differences. I'm not fully prepared to engage in a discussion of all of this -- it involves different concepts of man, and different consequences in terms of the doctrine of justification. But I've been following this excellent discussion, and I wanted to pass it along while it was still fresh in my mind.
It involves Rome's understanding of the donum superadditum with regard to the state that Adam was in before he fell. Briefly, Protestants say that man was "very good," and when he fell, it resulted in a spiritual death, and hence, God's imputation of righteousness and union with Christ is what's required to restore man back to his pristine, pre-fall state. Rome, on the other hand (thanks to Augustine's neopolatonism and some refinement by medieval theologians), holds that, not only was Adam "very good," but that he had some "super-added gift of grace" that he lost in the fall, and it is that "superadded grace" that needs to be restored. Hence the "infusion of grace" in Roman doctrine. [This accounts for the difference in which Calvin described man as "spiritually dead," and hence "totally inable" or "totally depraved," whereas Roman Catholics merely believe that man was "impaired" but with an otherwise full capacity to please God with his own grace-assisted works.]
Peter describes it this way:
For us, man originally had connatural beatitude, and when he fell, the reduced and superficial participation in that beatitude still possible to him, in an extrinsic way, was what we call temporal felicity or civic righteousness. But for RC, original felicity was a donum superadditum, and the status of original creation was thus left unclear, with at the very least a strong suggestion that much of what we think of as creation is in fact the effect of the Fall- an anti-Hebraic gnosticism which marred the thought of the ancient Greek church (and modern EO), and Rome too, though a more Biblical countertendency was present in the West and finally came into full victory with the Reformation. Given that the RC think of the New Covenant as the restoration of the donum superadditum, its relation to the temporal is ambivalent at best and hostile at worst. But for us, the New Covenant a) disables the heteronomous and unattainable Law which measured our alienation, and b) grants full citizenship in the Kingdom of God, simply by trust in Christ and union with Him. This means that the reality of original beatitude is poured into the forms of the creational order, and slowly transforms it spiritually, until all things shall be made new.There are many, many concepts tied up in this one little paragraph, and it would (and will) take a long time to extract the meaning from them. But the one I want to focus on is that, right from the outset of their understanding of man, Protestants and Catholics differ.
John Fesko, in his "Justification," describes this situation with respect to the "donum superadditum":
It seems as though much of the debate over infused versus imputed righteousness hinges upon the presuppositions of each party. The typical Reformed understanding is that Adam was created upright, or righteous, and that God justified, or declared righteous, the initial creation as well as man in his declaration that everything was "very good" (Gen 1:31). We see the Westminster Larger Catechism echo this point when it states that God created man in "righteousness, and holiness, having the law of God written in their hearts, and the power to fulfill it" (q. 17). By way of contrast, the typical Roman Catholic understanding of Adam's original state holds to the necessity of infused righteousness. Roman Catholic theologians typically hold to the idea of the donum superadditum ("superadded gift"). Medieval Roman Catholic theologians, for example, argue that the donum superadditum was a part of the original constitution of man, that it represented his original capacity for righteousness. We see then, from the outset, that man in his fallen state required infused righteousness in the form of the donum superadditum. If man requires infused righteousness in the prefall state, then he would most assuredly require it in his sin-fallen but redeemed state. The original state of man, then, is an issue that must feature in any dialogues over the question of imputation (John Fesko "Justification: Understanding the Classic Reformed Doctrine" Phillipsburg: P&R Publishing Company, pg 372.)Michael Horton, too, in his "Covenant and Salvation," expands on this concept greatly. "According to the federal theologians, Adam and Eve were never in a state of grace before the fall. Endowed in their creation with all of the requisite gifts for fulfilling God's eschatological purposes, there was nothing lacking requiring a gracious supplement" (194).
If anyone is interested, Horton develops this topic quite extensively, relying on Bavinck's account.
I've looked into this to some degree, and my hope is to discuss it in great detail once I've worked my way through the early church. But it is one of the key differences, and it is one of the distinctions that the Reformers made -- a (very likely unwitting) Roman misunderstanding and accretion that somehow became the law of the land at Trent.