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.
He does not speak here of the constancy of men, but of God, whose election must be ratified. He does not then, without reason declare, that where the calling of God is effectual, perseverance would be certain. He, in short, means that they who fall away had never been thoroughly imbued with the knowledge of Christ, but had only a light and a transient taste of it. 