John Calvin Commentary 1 John 2:19

"They went out from us, but they were not of us; for if they had been of us, they would have continued with us: but [they went out], that they might be made manifest that they all are not of us." — 1 John 2:19 (ASV)

They went out from us. He anticipates another objection: that the Church seemed to have produced these pests and to have cherished them for a time in its bosom. For it certainly is more disturbing to the weak when anyone among us, professing the true faith, falls away, than when a thousand outsiders conspire against us. He then confesses that they had gone out from the bosom of the Church, but he denies that they were ever of the Church. But the way of removing this objection is to say that the Church is always exposed to this evil, so that it is compelled to bear with many hypocrites who do not truly know Christ, however much they may profess his name with their mouths.

By saying, They went out from us, he means that they had previously occupied a place in the Church and were counted among the godly. He, however, denies that they were of us, though they had assumed the name of believers, just as chaff, though mixed with wheat on the same threshing floor, is not yet considered wheat.

For if they had been of us. He plainly declares that those who fell away had never been members of the Church. And undoubtedly, the seal of God, under which he keeps his own, remains sure, as Paul says (2 Timothy 2:19). But here a difficulty arises, for it happens that many who seemed to have embraced Christ often fall away.

To this I answer that there are three kinds of those who profess the Gospel:

1. There are those who feign piety, while a bad conscience condemns them from within.
2. The hypocrisy of others is more deceptive; these individuals not only seek to disguise themselves before men but also dazzle their own eyes, so that they seem to themselves to worship God rightly.
3. The third kind are those who have the living root of faith and hold a testimony of their own adoption firmly fixed in their hearts.

The first two kinds have no stability. John speaks of the last kind when he says that it is impossible for them to be separated from the Church, for the seal which God’s Spirit engraves on their hearts cannot be obliterated; the incorruptible seed, which has taken root, cannot be pulled up or destroyed.

He does not speak here of human constancy, but of God’s, whose election must be ratified. Therefore, he rightly declares that where the calling of God is effective, perseverance will be certain. In short, he means that those who fall away had never been thoroughly imbued with the knowledge of Christ, but had only a light and transient taste of it.

That they might be made manifest. He shows that trial is useful and necessary for the Church. It therefore follows, on the other hand, that there is no just cause for disturbance. Since the Church is like a threshing floor, the chaff must be blown away so that the pure wheat may remain. This is what God does, when he casts out hypocrites from the Church, for he then cleanses it from refuse and filth.