1 John 2:19

They went out from us
Which intends not the persons that went down from Judea to Antioch, ( Acts 15:1 Acts 15:24 ) , who preached destructive doctrines to the Gentiles, which the apostles and the church of Judea disowned and censured; by which it appeared, that all the preachers of these doctrines were not of them, and of the same mind with them: for this sense makes these antichrists to be only preachers; whereas, though many of them might be such, yet not all; for whoever, in a private capacity denied the Father and the Son, or that Christ was come in the flesh, was antichrist; and to these private believers are opposed in ( 1 John 2:20 ) ; and it also makes the "us" to be the apostles, whereas they were all dead but John; and these antichrists were men that had risen up then in the last time, and therefore could not, with propriety, be said to go out from the apostles; besides, whenever the apostle uses this pronoun "us", he includes with himself all true believers, and may more especially here intend the churches of Asia; or rather the members of the church at Ephesus, where he was; nor is it likely he should have in view the church of Judea, and a case in which that was concerned near forty years ago: moreover, such a sense makes the going out to be merely local and corporeal, and which is in itself not criminal; the persons that went from Judea to Antioch were not blamable for going thither, nor for going out from the apostles thither, but for troubling the disciples with words, to the subverting of their souls; nor was a corporeal departure from the apostles any evidence of not being of the same mind with them; for they often departed one from other, yet continued of the same mind, and in the same faith: but the sense is, that there were some persons in the Apostle John's time, who had made a profession of religion, were members of the church, and some of them perhaps preachers, and yet they departed from the faith, and dropped their profession of it, and withdrew themselves from the church, or churches to which they belonged, and set up separate assemblies of their own:

but they were not of us:
they were of the church, and of the same mind with it, at least in profession, antecedent to their going out; for had they not been in communion with the church, they could not be properly said to go out of it; and if they had not been of the same mind and faith in profession, they could not be said to depart from it; but they were not truly regenerated by the grace of God, and so apparently were not of the number, of God's elect: notwithstanding their profession and communion with the church, they were of the world, and not of God; they were not true believers; they had not that anointing which abides, and from which persons are truly denominated Christians, or anointed ones:

for if they had been of us, they would [no doubt] have continued
with us;
in the doctrine of the apostles, and in the fellowship of the church, as true believers do: if their hearts had been right with God, they would have remained steadfast to him, his Gospel, truths, and ordinances, and faithful with his saints; for such who are truly regenerate are born of an incorruptible seed, and those that have received the anointing which makes them truly Christians, that abides, as does every true grace, faith, hope, and love; and such who are truly God's elect cannot possibly fall into such errors and heresies as these did, and be finally deceived, as they were:

but [they went out];
"they went out from us", so the Syriac version reads;

that they might be made manifest that they were not all of us;
the word "all" is left out in the Syriac version. The defection and apostasy of these persons were permitted by God, that it might appear they had never received the grace of God in truth; and their going out was in such a manner, that it was a certain argument that they were not of the elect; since they became antichrists, denied the deity or sonship of Christ, or that he was come in the flesh, or that he was the Christ, and therefore are said to be of the world, and not of God, ( 1 John 2:22 ) ( 1 John 4:1 1 John 4:3-6 ) , so that this passage furnishes out no argument against the saints' perseverance, which is confirmed in ( 1 John 2:20 ) .