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.

From this verse it is plain that the "antichrists" were Christians who had defected from the truth. Their departure from the apostles and from the church indicated their hostility to the truth. Many of these were no doubt teaching the most shameful errors, justifying, or rationalizing the most wicked and dissolute behavior on the basis of Gnostic or other false teachings they had adopted.

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."