I did not write to you that you do not know the truth but that you know it and that all lying is not from the truth. 22 Who is the liar if not the one who renounces that Jesus is not the Christ? Such a man is the antichrist, the one who denies the Father and the Son. 23 All who deny the Son do not have the Father, the one who confesses the Son also has the Father.
I think it is interesting here that John goes out of his way to state that all lying is not from the truth, the truth being, of course, Jesus the Christ. Thus, by stating something as true that is not, one shows how Jesus is NOT living in them. Lying is the opposite of being a Christian.
This is a key practical point, since many Christians think white lies are justified for the greater good. There is a story in Exodus about how the midwives lied to Pharaoh concerning the Jewish women giving birth before they arrived. The text says that the women were bless, but it does not attach the blessing to their lies but rather to the results, that God’s people prospered. This is used by some people to say that lying is justified. However, John here makes it perfectly clear that no lie can ever come from Jesus or those who claim to follow Him.
The greatest liar is the one who denies Jesus, especially that Jesus is the Christ, the Anointed of God. Denying Jesus denies also the Father, and thus Judaism is no longer accurate as a religion since the Messiah has come and gone to be with the Father. Denying Jesus denies the true course of Judaism.
In the end, confession is not a one time act. John does not say that if you confess you are done. Rather, he speaks of confessing the Son, an ongoing action with ramifications for daily life. Sorry, but you do not get to pray the prayer and move on with life, instead you pray the prayer and give over your life.