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 it might become plain that they all are not of us.
—1 John 2:19

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

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 Tim 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:

  1. There are those who feign piety, while a bad conscience reproves them within.
  2. 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.
  3. 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. …

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.

—John Calvin
Commentaries on the Catholic Epistles, 191-192.