14 Nay, and that you may know that he has referred the matter to deeds, he says, “And every spirit, qui solvit Christum, which does away with Christ that He came in the flesh, is not of God.” A doing away in deeds is meant. What has he shown you? “That denies:” in that he says, “does away” (or, “unmakes”). He came to gather in one, you come to unmake. You would pull Christ's members asunder. How can it be said that you deny not that Christ has come in the flesh, who rendest assunder the Church of God which He has gathered together?
Therefore you go against Christ; you are an antichrist. Be thou within, or be thou without, you are an antichrist: only, when you are within, you are hidden; when you are without, you are made manifest. Thou unmakest Jesus and deniest that He came in the flesh; you are not of God. Therefore He says in the Gospel: “Whoso shall break one of these least commandments, and shall teach so, shall be called least in the kingdom of heaven.” What is this breaking? What this teaching? A breaking in the deeds and a teaching as it were in words. “Thou that preachest men should not steal, do you steal?” Therefore he that steals breaks or undoes the commandment in his deed, and as it were teaches so: “he shall be called least in the kingdom of heaven,” i.e. in the Church of this present time. Of him it is said, “What they say do ye; but what they do, that do not ye. But he that shall do, and shall teach so, shall be called great in the kingdom of heaven.”
From this, that He has here said, fecerit, “shall do,” while in opposition to this He has there said solverit, meaning non fecerit, “shall not do, and shall teach so”— to break, then, is, not to do— what does He teach us, but that we should interrogate men's deeds, not take their words upon trust? The obscurity of the things compels us to speak much at length, chiefly that that which the Lord deigns to reveal may be brought within reach even of the brethren of slower understanding, because all were bought by the blood of Christ. And I am afraid the epistle itself will not be finished during these days as I promised: but as the Lord will, it is better to reserve the remainder, than to overload your hearts with too much food.
Source: Homilies on the First Epistle of John (New Advent)