Whoever denies or distorts the true teaching concerning Jesus Christ as the Son of God is an antichrist, a deceiver, or a false teacher. He is filled with the spirit of antichrist, like one of antichrist's forerunners, of whom many have already appeared and many more will appear.
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 form of expression is such that he 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.
"Who denies the Father and the Son." He is an antichrist, an opponent of Christ, because he denies Christ. Yet at the same time he also denies God the Father. Therefore he denies the Father and the Son.
The Apostle then explains why such a man, by denying the Son, also denies the Father. The true God is known only in the Son and through the Son. No other communion with God is possible except through His Son.
Therefore whoever has a right understanding of the Son of God rightly believes in God and honors Him. Whoever has a false understanding of the Son believes in God falsely and honors Him falsely.
Or again, whoever believes in Jesus Christ also believes in God the Father. Whoever denies the One denies the Other also.