4 Whom has he called antichrists? He goes on and expounds. “Whereby we know that it is the last hour.” By what? Because “many antichrists have come. They went out from us;” see the antichrists! “They went out from us:” therefore we bewail the loss. Hear the consolation. “But they were not of us.” All heretics, all schismatics went out from us, that is, they go out from the Church; but they would not go out, if they were of us. Therefore, before they went out they were not of us. If before they went out they were not of us, many are within, are not gone out, but yet are antichrists. We dare to say this: and why, but that each one while he is within may not be an antichrist? For he is about to describe and mark the antichrists, and we shall see them now. And each person ought to question his own conscience, whether he be an antichrist. For antichrist in our tongue means, contrary to Christ. Not, as some take it, that antichrist is to be so called because he is to come ante Christum, before Christ, i.e. Christ to come after him: it does not mean this, neither is it thus written, but Antichristus, i.e. contrary to Christ. Now who is contrary to Christ you already perceive from the apostle's own exposition, and understand that none can go out but antichrists; whereas those who are not contrary to Christ, can in no wise go out. For he that is not contrary to Christ holds fast in His body, and is counted therewith as a member. The members are never contrary one to another. The entire body consists of all the members. And what says the apostle concerning the agreement of the members? “If one member suffer, all the members suffer with it; and if one member be glorified, all the members rejoice with it.” If then in the glorifying of a member the other members rejoice with it, and in its suffering all the members suffer, the agreement of the members has no antichrist. And there are those who inwardly are in such sort in the body of our Lord Jesus Christ— seeing His body is yet under cure, and the soundness will not be perfect save in the resurrection of the dead— are in such wise in the body of Christ, as bad humors. When these are vomited up, the body is relieved: so too when bad men go out, then the Church is relieved. And one says, when the body vomits and casts them out, These humors went out of me, but they were not of me. How were not of me? Were not cut out of my flesh, but oppressed my breast while they were in me.
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.
6. “I write unto you not because you know not the truth, but because you know it, and that no lie is of the truth.” Behold, we are admonished how we may know antichrist. What is Christ? Truth. Himself has said “I am the Truth.” But “no lie is of the truth.” Consequently, all who lie are not yet of Christ. He has not said that some lie is of the truth, and some lie not of the truth. Mark the sentence. Do not fondle yourselves, do not flatter yourselves, do not deceive yourselves, do not cheat yourselves: “No lie is of the truth.” Let us see then how antichrists lie, because there is more than one kind of lying. “Who is a liar, but he that denies that Jesus is the Christ?” One is the meaning of the word “Jesus,” another the meaning of the word “Christ:” though it be one Jesus Christ our Saviour, yet “Jesus” is His proper name. Just as Moses was so called by his proper name, as Elias, as Abraham: so as His proper name our Lord has the name “Jesus:” but “Christ” is the name of His sacred character. As when we say, Prophet, as when we say, Priest; so by the name Christ we are given to understand the Anointed, in whom should be the redemption of the whole people. The coming of this Christ was hoped for by the people of the Jews: and because He came in lowliness, He was not acknowledged; because the stone was small, they stumbled at it and were broken. But “the stone grew, and became a great mountain;” and what says the Scripture? “Whosoever shall stumble at this stone shall be broken; and on whomsoever this stone shall come, it will grind him to powder.” We must mark the difference of the words: it says, he that stumbles shall be broken; but he on whom it shall come, shall be ground to powder. At the first, because He came lowly, men stumbled at Him: because He shall come lofty to judgment, on whomsoever He shall come, He will grind him to powder. But not that man will He grind to powder at His future coming, whom He broke not when He came. He that stumbled not at the lowly, shall not dread the lofty. Briefly you have heard it, brethren: he that stumbled not at the lowly, shall not dread the lofty. For to all bad men is Christ a stone of stumbling; whatever Christ says is bitter to them.
Source: Homilies on the First Epistle of John (New Advent)