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.
Such a verse as this, of course, is made use of as a crutch for the proposition that a person "once saved is always saved"; however, it should be carefully noted that John did not here write of the false teachers that "they never had been of us," but that at an unspecified previous time, they were not.  This is even more clear in the last clause where the word is not that they had never been of us, but that they are not of us.  Their departure from the faith became final at some point prior to their leaving; but there is no suggestion by the apostle that those who departed had never been truly converted at the beginning of their Christian association.  The fallen angels were not wicked from the beginning but became so; and Judas was not wicked when the Lord chose him as an apostle, but he fell "through transgression."