1 John 2:19; 1 John 4:1

19 They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, so that it would be shown that they all are not of us.
1 Beloved, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world.