There is an exceptional discussion of justification in the early church over at Triablogue: Triablogue: Seeds Of The Reformation This is part of an ongoing discussion with some of the “Called to Communion” gang. As many of you probably know, justification was at the root of the Reformation. But what many people, even Reformed believers, don’t understand is that the Roman Catholic conception of justification does not have its roots in the Scriptures; rather, they have their beginnings in Aquinas’s use of Aristotelian categories – not Scripture – to describe such things as God, reason, and grace. J.V. Fesko, in his excellent work “Justification,” describes quite accurately where these discrepancies come from, the effect they have on the Roman doctrine, and also some of the consequential outworkings. (I’ve posted selections from Fesko at the Triablogue discussion, but this is something that everyone should understand). Amazon.com: Justification: Understanding the Classic Reformed Doctrine (9781596380868): J. V. Fesko: Books (Michael Horton also writes about this in more detail in Covenant and Salvation and also in People and Place). As a practical out-working then, my understanding is that there are several key differences which don't get mentioned in these types of discussions: "Grace" actually means different things when Catholics and Protestants talk about it. So when Catholics say that “we both affirm that justification and salvation are by ‘grace alone,’” they fail to acknowledge that there is equivocation on this word “grace.” This is reflected from the very beginning; in the Protestant schema, man was "very good" when God created him. So Christ's sacrifice, for Protestants, brings man through forensic justification, a declarative act of God, restoring man back to the "very good" that he had "in the beginning." In the Catholic scheme, man as created wasn't yet "good enough." He was merely neutral. There was this "super-added Grace" that gave him a kind of supernatural character that enabled him to "fellowship with God." So in Roman Catholics, this begins what I call the “sacramental treadmill” -- beginning with an "infusion" of grace (a little squirt of this good oil that is yours to maintain by staying in "a state of grace") which does "make you righteous," but then it must be maintained through life -- being a "good Catholic," it can be "added to" (through the "increase of merit"). This is required not by God in the Word, but by the Roman Magisterium, which lost the proper understanding of justification because of Augustine’s errors, and which found new life through Rome’s need to affirm that it has “never erred.” Such errors are propped up by the Aristotelian/Thomist necessity to maintain human reason as a principle above and controlling God's Word. It should be noted, too, that Aquinas made other errors, and incorporated them into his theology. He believed that “Pseudo-Dionysius,” a fifth century neo-Platonist, was actually “Dionysius the Areopagite” from Acts 17. As well, he accepted many of the forgeries of the middle ages as if they were genuine items. Sorry if this is a bit long, but Fesko explains it very, very clearly. There is much more to it (including longer explanation from Van Til about the outworkings), but the gist of it is that Roman Catholics, with their Aristotelian understanding of God, really do not have the God of the Bible in mind when they, in their very core doctrines, use Aristotle to understand him.