One of the classic works on this question was written by a man by the name of James Buchanan. It was simply titled Justification, which addressed the chief issue of the Protestant Reformation of the sixteenth century. One of the great values of Buchanan’s contribution is that he traced the doctrine of justification by faith alone through the patristic period and all the way through church history, showing that it was never totally extinguished within the Roman system that had radically departed from the biblical doctrine. So, that information is available.
This is a critical matter. I wrote a book recently titled Are We Together? We’re not. Rome has never changed her view of the condemnation of the gospel of justification by faith alone that occurred at the sixth session of the Council of Trent in the sixteenth century. That condemnation was repeated as recently as the Catholic Catechism in the 1990s.
In the Tridentine view of the sixteenth century, not only were there canons against the Protestant view, but the beginning part of the session carefully articulated the Roman understanding of justification, or what we would call the Roman understanding of the gospel. If you compare Trent’s understanding of the gospel with the Reformation understanding of the gospel, you have to see that they cannot both be right. One of them has to be a false gospel, and only one of them can be the true gospel. That’s what the issue was then, and that issue continues even now.