Therefore the Apostle does not enter into details. He points only to the surest and most decisive mark of false teaching. "Who is the liar, if not he who denies that Jesus is the Christ? This is the antichrist, who denies the Father and the Son."
In other words, the liar, false teacher, and antichrist in the general sense is the one who denies or distorts the teaching concerning the divine or human nature of Jesus Christ. Such a man does not acknowledge that Jesus of Nazareth is the Messiah, the Son of God and Savior of the world, the God-man, consubstantial with God the Father.
The Apostle's form of expression means that such a man is not merely a liar, but the liar above all others: "Who is the liar, if not he who denies?" There may be liars of various kinds, who deny or distort one teaching or another, one dogma or another.
But the one who denies or distorts the fundamental and essentially important teaching concerning the God-manhood of Jesus Christ denies the chief cornerstone in the building of the Church. He is the liar above all others, a liar of liars, an antichrist or a son of antichrist.