Peter truly speaks as the victor of Rome when he said that Paul's Doctrine of Faith Alone was difficult to understand. This is an Essene pesher when I equate Peter with Roman Catholicism and Paul with Protestantism, but is it not true that the Tridentine formulas that are so odious to Protestants really are due to this same problem of Peter's confession regarding the great difficulty in studying the doctrine of Grace. Continuing my pesher, there is also a warning that Peter pronounces after his confession that we are in danger and many have already done so to twist the Doctrine of Grace to our own destruction and of others. Beware!
2 Peter 3:15-18 NRSV "So also our beloved brother Paul wrote to you according to the wisdom given him, 16 speaking of this as he does in all his letters. There are some things in them hard to understand, which the ignorant and unstable twist to their own destruction, as they do the other scriptures. 17 You therefore, beloved, since you are forewarned, beware that you are not carried away with the error of the lawless and lose your own stability. 18 But grow in the grace and knowledge of our Lord and Savior Jesus Christ. To him be the glory both now and to the day of eternity."
Hans Küng has demonstrated that the Tridentine statements on grace are wrongly misunderstood by Protestants to be Pelagian or Semi-Pelagian synergism. Despite the explicit denial of Semi-Pelagianism in the Second Council of Orange, there is no convincing some Protestants otherwise (and some Catholics as well!) despite Peter's warning (2 Peter 3:17).
A point of particular difficulty to understand, which continues to fuel the Protestant Reformation, is the Pauline vs Petrine understanding of Grace. In the following quotation from Hans Küng's Doctrine of Justification, is an excellent explanation of how the Roman Catholic doctrine of Grace, i.e. habitus, is understand by Catholics and misunderstood.
"Barth's fears that God's grace might become, perniciously, 'my' grace are unfounded if we keep in view the fact that grace is mine only as the grace of God; I never "have" it; it is never simply at my disposal. The term habitus is not meant in the sense of "having" grace, but, as Bonaventure explains "to hold is to be held" [..]. Grace is given to me each day as something completely new. It becomes "my" grace--as a consequence of the incarnation--but always as a grace alien to me, according to the paradoxical formulation of Trent: [..] ("Thus, it is not personal effort that makes justice our own."--D809). The 'Index of Celestine' states in Chap. 2: "Unless he who alone is good grants a participation in his being, no one has goodness within himself. This truth is proclaimed by that prontiff (Innocent I) in the following sentence: 'For the future, can we expect anything good from those who mentality is such that they think they are the cause of their goodness and do not take into account him whose grace they obtain each day, and who hope to accomplish so much without him?" And in Chap. 6: "The same teacher Zosimus instructed us to acknowledge this truth when, speaking to the bishops of the world about the assistance of divine grace, he said: 'Is there ever a time when we do not need his help? Therefore, in every action and situation, in every thought and movement, we must pray to him as our helper and protector'" (D 131 and 135)"
Küng, Hans Justification: The Doctrine of Karl Barth and a Catholic Reflection. Louisville, KY: Westminster John Knox, 2004. 205. Print.
I believe in One Holy Catholic Church.
Related: Doctrine of Justification, Hans Küng, Peter and Paul, Second Council of Orange, Tridentine