All Catholics should imitate St John in rejecting the false Church
The story is recorded in the second century book Against Heresies by St. Irenaeus. Irenaeus had been a disciple of St. Polycarp, who had been a disciple of John. It’s from this direct line to John that Irenaeus got his information.
At the end of the first century, there was a gnostic heretic named Cerinthus. Among other things, he denied the Virgin Birth, denied that Jesus was the Christ his whole life, and taught that Christians were required to follow the Mosaic Law.
One day, John went inside a public bathhouse, but quickly spotted Cerinthus inside. John immediately ran out of the building, exclaiming to those with him, “Let us fly, lest even the bath-house fall down, because Cerinthus, the enemy of the truth, is within!” (Against Heresies, 3.3.4)
The same saint wrote the Book of Apocalypse. In Apocalypse 18:4 he writes: "And I heard another voice from heaven, saying: Go out from her, my people; that you be not partakers of her sins, and that you receive not of her plagues."
Catholics should heed this warning and come out of the false church set up in the Vatican. This warning is from the very saint whose feast day is today.