1 John 2:18-19
Little children, it is the last hour and as you have heard that the Anti-Christ is coming, even now many anti-christs have come, by which we know that it is the last hour. 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 manifest, that none of them were of us.
This passage has been held up by Calvinists as teaching a universal principal of what “apostasy” constitutes. Remember, apostasy in Calvinism means only that one rejects the gospel. It is not a falling away from true faith. True faith endures since God infallibly preserves it and if one should seem to fall away it only proves that the person never possessed genuine faith and had never been regenerated. It is impossible, according to Calvinism, to fall away from true faith. One can only fall away from a false profession of true faith and prove that one was always just a hypocrite. Calvinists believe that this single passage of Scripture proves their strange definition of apostasy to be the Biblical definition.
John speaks of false teachers (anti-christs) who went out “from us” (the true gospel teachers) and thus proved by their going that they were not “of us.” Does John then teach the Calvinistic definition of apostasy? Not at all. The passage simply does not say what the Calvinist needs it to say for several reasons.
First, John is not laying down a universal principle concerning what it means to be an apostate. John is specifically speaking of false teachers leaving the company of the true gospel teachers and proving by their leaving that they are not in harmony with the true gospel. Had they continued in the truth they would have no reason to leave but since they had abandoned the truth they could no longer keep company with the true gospel teachers and went out from them to spread their heresies. By doing so they proved that their authority is not from God, and their teachings should not be trusted. This is John’s primary point in this passage.
Second, the passage says nothing of the false teachers’ prior spiritual condition. It only tells us that at the time of their going they were not “of” the true gospel teachers. They were committed to false doctrine when they left and left for that reason, but we have no way of knowing whether or not they had at one time genuinely embraced the truth. The passage does not address their spiritual condition prior to their leaving, and this is exactly what the Calvinist needs the passage to do in order to support their doctrine. The Calvinist needs the passage to say, “They went out from us, but they were not [ever] of us; for if they had [ever] been of us, they would have continued with us; but they went out that they might be manifest, that none of them were [ever at any time] of us.” The “ever” must be read into the text. It is simply not there and the Calvinist must beg the question to assume that this is what John meant to imply. For this reason alone, this passage fails as a proof text for Calvinistic apostasy.
Third, the context of the epistle argues for the view that these false teachers were indeed saved prior to their defection and left only after embracing false teaching and thereby apostatizing from the truth they once embraced. One of the main issues being addressed throughout 1 John is how one can determine whether or not one is truly saved (“born of God”). The Gnostics (i.e. anti-christs) were teaching that there was no connection between behavior and salvation. They believed that the human spirit was incorruptible and could in no way be affected by the sins of the flesh. John directly opposes such teaching numerous times in his epistle (1:5-10; 2:1, 3-6, 9-11, 15; 3:4-11, 15, 17, 18, 24; 4:7, 16, 20, 21; 5:1, 2). John is primarily encouraging his readers to reject the false teachings of the “anti-christs” who are teaching that one can sin with spiritual immunity, and helping them to understand the true characteristics of God’s children.
Now for us to believe that the anti-christs who left the company of the true gospel teachers were never true believers would suggest that John and the true gospel teachers were not able to detect their hypocrisy while they kept their company. This runs contrary to one of John’s main concerns in the epistle, that one can discern the difference between true followers of Christ and unbelievers by character and behavior. To believe that John was incapable of detecting their hypocrisy prior to their actual defection is out of harmony with one of the most prominent themes of the entire letter.
From: Perseverance of the Saints Part 12: Examining Passages Commonly Appealed to by the Advocates of Unconditional Eternal Security