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)