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.

 As things have turned out, John sees in the Church a time of sifting. The false teachers had voluntarily left the Christian fellowship; and that fact had shown that they did not really belong there. They were aliens and their own conduct had shown it to be so. 