They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.
Out from us from our Christian communion Not necessarily a formal secession; thus Rome has spiritually Out from us - from our Christian communion. Not necessarily a formal secession; thus Rome has spiritually gone out, though formally still of the Christian Church.
Not of us - by spiritual fellowship (1 John 1:3): 'like bad humours in the body of Christ: when they are vomited out, the body is relieved; it is still under treatment, and has not yet attained the perfect soundness which it shall have only at the resurrection' (Augustine, 'Ep. John, Tract,' 3,4).
They would ... have continued - implying the indefectibility of grace in the elect. 'Where God's call is effectual, there will be sure perseverance' (Calvin). No fatal, but a 'voluntary necessity' (Didymus), causes men to remain, or else go from the body of Christ. 'We are among the members, or else among the bad humours. It is of his own will that each is either an Antichrist, or in Christ' (Augustine). God's eternal election harmonizes in a way inexplicable to us, with man's free agency. It is men's own evil will that chooses hell; it is God's sovereign grace that draws anyone to heaven. To God the latter shall ascribe wholly their salvation; the former shall reproach themselves alone, not God's decree, with their condemnation (1 John 3:9; 1 John 5:18).
That they were set all of us. This would imply that some of the Antichrists are of us! Translate, "That all (who are among us) ARE not of us." Compare 1 Corinthians 11:19. Such occasions test who are, and who are not, the Lord's people.