There was a Black Mass performed in the Vatican complex in 1963, to enthrone Lucifer there. Paul VI stated that the smoke of Satan had entered into the Vatican. This is what he was referring to. If I remember correctly, there were concurrent Black Masses performed in certain Catholic cathedrals in the US - I believe Boston, South Carolina, and LA were all mentioned.
The gates of hell will not prevail, but it will be a close run thing.