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.
Source: Homilies on the First Epistle of John (New Advent)