Not the most abused religious quote, but it is one frequently co-opted by those who wish to bolster a false narrative:
“[F]rom some fissure the smoke of Satan has entered the temple of God.”
This quote is apparently taken from of a homily given by Pope Paul VI in 1972. The source for the quote is not even a verbatim copy of the homily itself but a “rendering” of it. We cannot know for certain what it is Pope Paul VI had said or the precise context, but, if the narrative is accurate, he was speaking of an evil force in society, from the 1960s to the 1970s, that has caused doubt and confusion among the faithful towards the Church despite the renewals enacted by the Second Vatican Council. Paul VI was not referring to some sinister infiltration of the Church during the Council proceedings but societal upheavals resulting from so-called advances in progress made in the modern world. And because of this, “There is no longer trust of the Church; they trust the first profane prophet who speaks in some journal or some social movement, and they run after him and ask him if he has the formula of true life.”
Later in the homily, Paul VI explains further by blaming the devil, who has had a perfidious influence on the Church and the world:
“We believe in something that is preternatural that has come into the world precisely to disturb, to suffocate the fruits of the Ecumenical Council, and to impede the Church from breaking into the hymn of joy at having renewed in fullness its awareness of itself. Precisely for this reason, we should wish to be able, in this moment more than ever, to exercise the function God assigned to Peter, to strengthen the Faith of the brothers. We should wish to communicate to you this charism of certitude that the Lord gives to him who represents him though unworthily on this earth.”
The answer Paul VI gives to the threat Satan poses is not a “reform of the reform” or “clarification” of passages in the Vatican II documents that some find ambiguous or a renunciation of Vatican II altogether, but a call to be “strong in faith.” It is through faith in God and his Church that we’re given the strength and certitude to “triumph over all obstacles” and be at peace.
The full text of the homily, translated into English, is provided by Jimmy Akins at his website, along with his commentary on it.