5 “They went out from us; but,” be not sad, “they were not of us.” How do you prove this? If they had been of us, they would doubtless have continued with us. Hence therefore you may see, that many who are not of us, receive with us the Sacraments, receive with us baptism. receive with us what the faithful know they receive, Benediction, the Eucharist, and whatever there is in Holy Sacraments: the communion of the very altar they receive with us, and are not of us.
Temptation proves that they are not of us. When temptation comes to them as if blown by a wind they fly abroad; because they were not grain. But all of them will fly abroad, as we must often tell you, when once the fanning of the Lord's threshing-floor shall begin in the day of judgment. “They went out from us, but they were not of us; if they had been of us, they would no doubt have continued with us.” For would you know, beloved, how most certain this saying is, that they who haply have gone out and return, are not antichrists, are not contrary to Christ?
Whoso are not antichrists, it cannot be that they should continue without. But of his own will is each either an antichrist or in Christ. Either we are among the members, or among the bad humors. He that changes himself for the better, is in the body, a member: but he that continues in his badness, is a bad humor; and when he is gone out, then they who were oppressed will be relieved. “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.”
That he has added, “that they might be made manifest,” is, because even when they are within they are not of us; yet they are not manifest, but by going out are made manifest. “And you have an unction from the Holy One, that you may be manifest to your own selves.” The spiritual unction is the Holy Spirit Himself, of which the Sacrament is in the visible unction. Of this unction of Christ he says, that all who have it know the bad and the good; and they need not to be taught, because the unction itself teaches them.
Source: Homilies on the First Epistle of John (New Advent)