8 But let us not be made sad: “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.” If then they went out from us, they are antichrists; if they are antichrists, they are liars; if they are liars, they deny that Jesus is the Christ. Once more we come back to the difficulty of the question. Ask them one by one; they confess that Jesus is the Christ. The difficulty that hampers us comes of our taking what is said in the Epistle in too narrow a sense.
At any rate you see the question; this question puts both us and them to a stand, if it be not understood. Either we are antichrists, or they are antichrists; they call us antichrists, and say that we went out from them; we say the like of them. But now this epistle has marked out the antichrists by this cognizance: “Whosoever denies that Jesus is the Christ,” that same “is an antichrist.” Now therefore let us enquire who denies; and let us mark not the tongue, but the deeds. For if all be asked, all with one mouth confess that Jesus is the Christ.
Let the tongue keep still for a little while, ask the life. If we shall find this, if the Scripture itself shall tell us that denial is a thing done not only with the tongue, but also with the deeds, then assuredly we find many antichrists, who with the mouth profess Christ, and in their manners dissent from Christ. Where find we this in Scripture? Hear Paul the Apostle; speaking of such, he says, “For they confess that they know God, but in their deeds deny Him.” We find these also to be antichrists: whosoever in his deeds denies Christ, is an antichrist.
I listen not to what he says, but I look what life he leads. Works speak, and do we require words? For where is the bad man that does not wish to talk well? But what says the Lord to such? “You hypocrites, how can you speak good things, while you are evil?” Your voices you bring into my ears: I look into your thoughts. I see an evil will there, and you make a show of false fruits. I know what I must gather, and whence; I do not “gather figs of thistles,” I do not gather “grapes of thorns;” for “every tree is known by its fruit.” A more lying antichrist is he who with his mouth professes that Jesus is the Christ, and with his deeds denies Him. A liar in this, that he speaks one thing, and does another.
Source: Homilies on the First Epistle of John (New Advent)