Hello @Winslow89,
yes, I would say that this is true of v19, generally speaking, that is.
I have no more time to talk right now, but I will leave you with the following to consider (it's commentary on v19 from a favorite theologian of mine, John Stott).
19. The many antichrists who have already come (in contrast to the one antichrist who shall come) are now identified as human teachers. They are no doubt the same as the ‘many false prophets’ of
1 John 4:1 (cf. our Lord’s combination of ‘false Christs and false prophets’ in Mark 13:22). They have left the church to which John is writing, perhaps because they have failed to win over the church leaders to their viewpoint. They have excommunicated themselves. John distinguishes sharply between the they who have left and the us who remain. They went out (exēlthan) from us. By their defection they have given clear evidence of their true character. In fact, their withdrawal was ‘anticipated by Judas Iscariot, who “went out” (exēlthen) from the company of Jesus and the disciples in the upper room (John 13:30a …)’ (Smalley, p. 102). Just so, the false teachers did not really belong to us. So certain is John of this fact that he adds the hypothesis: if they had belonged to us, they would have remained with us.
John not only relates the fact of their departure from the fellowship, but discerns a purpose in it. The heretics went out of their own volition, but behind the secession was the divine purpose that the spurious should be ‘made manifest’ (AV) (phanerōthōsin) lest the elect should be led astray (Matt. 24:24). Their departure was ‘their unmasking’ (Law). What is counterfeit cannot remain for ever hidden (cf. 1 Cor. 3:13; 1 Cor. 4:5). ‘There is nothing concealed that will not be disclosed, or hidden that will not be made known’ (Luke 12:2).
Light is shed by this verse upon two important doctrines: the perseverance of the saints and the nature of the church. ‘He who stands firm to the end will be saved’ (Mark 13:13), not because salvation is the reward of endurance, but because endurance is the hallmark of the saved. If the false teachers had belonged to us, they would have remained with us. This is stated as a principle. Those who belong to us stay with us. Future and final perseverance is the ultimate test of a past participation in Christ (cf. Heb. 3:14). ‘Those who fall away’, on the other hand, ‘have never been thoroughly imbued with the knowledge of Christ but only had a slight and passing taste of it’ (Calvin).
This verse also gives biblical warrant for some distinction between the visible and the invisible church. Granted that God intends his church to be visibly manifest in local worshipping, witnessing fellowships, this does not mean that all the professing baptized, communicant members of the church are necessarily members of Christ. Only the Lord knows ‘those who are his’ (2 Tim. 2:19). Perhaps most visible-church members are also members of the invisible church, the mystical body of Christ, but some are not. They are with us yet do not really belong to us. They share our earthly company but not our heavenly birth. Only on the final day of separation will the wheat and the tares be completely revealed. Meanwhile, some are shown up in their true colours by their defection. Cf. 1 John 3:10 for a manifestation in moral behaviour. ~Stott, J. R. W. (1988). The Letters of John: An Introduction and Commentary (Vol. 19, pp. 108–110). InterVarsity Press.
God bless you!!
~Deuteronomy (David)