In his Crisis Magazine classic, James Hitchcock writes that while the Second Vatican Council was itself orthodox, much of what followed was not. I agree with all of his conclusions, except there is one he missed, given by Paul VI after the council.
On March 17, 1990, in Il Sabato magazine in Rome) the Holy Father told a congregation:
We have the impression that through some cracks in the wall the smoke of Satan has entered the temple of God: it is doubt, uncertainty, questioning, dissatisfaction, confrontation. And how did this come about? We will confide to you the thought that may be, we ourselves admit in free discussion, that may be unfounded, and that is that there has been a power, an adversary power. Let us call him by his name: the devil. We thought that after the Council a day of sunshine would have dawned for the history of the Church. What dawned instead was a day of clouds and storms, of darkness, of searching and uncertainties.
His fears of demonic penetration of the Church were even stronger in a later statement:
The opening to the world [aggiornamento or “updating”] became a veritable invasion of the Church by worldly thinking....We have perhaps been too weak and imprudent.
For an historical account of Hitchcock's article see here.