They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained with us (1 John 2:19a).
Notice that the mark of an antichrist is finally breaking away from New Testament Christianity. All such invariably do. And when they do, they will insist that they are the true mainstream of Christian truth and that we are living in the backwaters of Christian doctrine. John puts this very plainly. The mark of genuineness is continuity in the truth, continuance in true faith. What does he mean by
us? Surely not Christendom in general. He means, as he makes clear in the context of this whole letter, those who love the Word of God and who possess the Spirit of God, those who seek to obey the Word in the power of the Spirit. The emphasis he has been making all along is on those who share the life of Christ by the Word of God in the power of the Spirit of God. Heretics will invariably cut themselves off from these people.