1 John 2:19, 26

They went out from us [seeming at first to be Christians], but they were not really of us [because they were not truly born again and spiritually transformed]; for if they had been of us, they would have remained with us; but they went out [teaching false doctrine], so that it would be clearly shown that none of them are of us.

These things I have written to you with reference to those who are trying to deceive you [seducing you and leading you away from the truth and sound doctrine].