1 John 2:19
The proliferation of false teachers indicates to John that it is (the) last hour (no definite article). Note: the entire period between the first and second comings of Jesus = the "last days". See Acts 2:17; 2 Tim. 3:1; Heb. 1:2; 1 Pt. 1:20 (cf. 1 Cor. 10:11).
The antichrists of v. 18 = the false teachers against whom the epistle is directed. They are the ones whom John wishes to expose by means of the application of his "tests of life". Here in v. 19 he indicates that at one time they were "members" of the community which professed faith in Christ. They were actively involved in the ministry of the church and until the moment of separation were hardly distinguishable from the rest of the Christian society.
"they went out from us" - either by excommunication or voluntary separation (probably the latter); note the sharp distinction between "they" and "us"
"but they were not of us" - in spite of their external membership, they did not share the inner life: "of us" = the spiritual bond of the body of Christ
"for if they had been of us they would have remained with us" - Westcott writes: "If they had in the truest sense shared our life, the life would have gone forward to its fruitful consummation" (71). Again, we see that the test of life and salvation is abiding, persevering.
"but (they went out) in order that they might be made manifest that they all are not of us" - there is a divine purpose in their secession, namely, exposure of those who are but professors; their departure was their unmasking (cf. 1 Cor. 11:18-19)
Two things should be noted here:
(1) The doctrine of the perseverance of the saints - Abiding or continuance or endurance is the sign of the saved, just as apostasy is the evidence of initial unbelief. Note the emphasis of the phrase: "for if they had been of us, they would have remained with us . . ." (cf. Heb. 3:6,14). The presence of genuine faith ("of us") implies (necessitates) perseverance.
(2) The doctrine of the church in the present age - Stott writes: "Granted that God intends His church to be visibly manifest in local worshiping, 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 'them that 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 not of us. They share our earthly company but not our heavenly birth. Only on the final day of separation will the wheat and tares be completely revealed. Meanwhile, some are made manifest in their true colors by their defection" (106).