They went out from us, but they didn't belong to us; for if they had belonged to us, they would have continued with us. But they left, that they might be revealed that none of them belong to us.
for if they had been of us, they would no doubt have continued with us: Sometimes this statement is used to prove the impossibility of apostasy, or "once saved, always saved." Those who believe this doctrine say that once a person is saved he is eternally saved; and if he does prove himself to be lost, he was not saved in the first place. The circumstances are quite different here from the case of one who comes to Christ, believing in Him as the Son of God and obeying His word. In this situation, these people denied the very truths that made it possible for them to become Christians. Whatever steps they took did not make them Christians. If they had been "of us," they would have believed the basic truth that Christ came in the flesh to save man; and they would have remained faithful to Christ. Their belief that Jesus was just a man, and not divine, drove them from those who promoted this truth with vigor.