R. C. Sproul demonstrates the contradiction in Roman Catholic Theology, when it claims it agrees with Augustine against Pelagius and the Semi-Pelagians (Provincial Synod of Orange in 529 AD), but later re-affirms Semi-Pelagianism by the decrees of Trent (1545-1563) and then, later, arguably, it approves of even Pelagianism by the condemnation of the Jansenists (roughly, 1638-1713) and the modern Roman Catholic Catechism of 1994. Sproul calls it an "ambiguity". Indeed, it is more than that; it is a real contradiction. It also shows the Roman Catholic Church to be fallible; thus bringing down the whole system of its claim to be infallible.
I recommend Sproul's book, Willing to Believe, and DVD teaching series.
See also an earlier post about the tendency of Roman Catholicism to drift back to Semi-Pelagianism between the Council of Orange in 529 AD until Luther questioned the issue, and the Council of Trent in 1545-1563.