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 He plainly declares that those who fell away had never been members of the Church. And doubtless the seal of God, under which he keeps his own, remains sure, as Paul says, (2 Timothy 2:19.) But here arises a difficulty, for it happens that many who seemed to have embraced Christ, often fall away. To this I answer, that there are three sorts of those who profess the Gospel; there are those who feign piety, while a bad conscience reproves them within; the hypocrisy of others is more deceptive, who not only seek to disguise themselves before men, but also dazzle their own eyes, so that they seem to themselves to worship God aright; the third are those who have the living root of faith, and carry a testimony of their own adoption firmly fixed in their hearts. The two first have no stability; of the last John speaks, when he says, that it is impossible that they should be separated from the Church, for the seal which God’s Spirit engraves on their hearts cannot be obliterated; the incorruptible seed, which has struck roots, cannot be pulled up or destroyed. 