1 John 2:19

Verse 19. They went out from us] These heretics had belonged to our Christian assemblies, they professed Christianity, and do so still; but we apostles did not commission them to preach to you, for they have disgraced the Divine doctrine with the most pernicious opinions; they have given up or explained away its most essential principles; they have mingled the rest with heathenish rites and Jewish glosses. While, therefore, we acknowledge that they once belonged to us, we assert that they are not of us. They are not Christians; we abhor their conduct and their creed. We never sent them to teach.

They were not of us] For a considerable time before they left our assemblies they gave proofs that they had departed from the faith; for if they had been of us-if they had been apostles, and continued in the firm belief of the Christian doctrines, they would not have departed from us to form a sect of themselves.

That they were not all of us.] They were not expelled from the Christian Church; they were not sent out by us; but they separated from it and us. None of them had been inspired as we apostles were, though they pretended to a very high teaching; but their separating from us manifested that they were not taught, as we were, by the Spirit of God. These false teachers probably drew many sincere souls away with them; and to this it is probable the apostle alludes when he says, they were not ALL of us. Some were; others were not.