If John 3:13 states that no man ascended to heaven except Christ, then how come both Enoch in Genesis 5:24 and Elijah in II Kings 2:1 ascended?
"No one has ascended to heaven but He who came down from heaven, that is, the Son of Man who is in heaven" (John 3:13).
The wording may a bit odd to our way of thinking, but Jesus' point is that no one has gone to heaven and returned to tell about it. In other words, no one has ever been able to speak with first-hand knowledge about God and the heavenly realm, except for Christ himself who originated in heaven. Unlike any man, Jesus is stating that his native abode is heaven and not the earth. He is also stating he existed before he was born here on earth.
Jesus is alluding to what Moses said, "It is not in heaven, that you should say, 'Who will go up to heaven for us to get it for us and make us hear it, that we may observe it?'" (Deuteronomy 30:12). Moses said that the commands from heaven were not impossible to follow because no one was required to do the impossible to fetch them. Jesus is making a similar point. He is bringing the commands from God directly, something he is uniquely able to do and not any man.
The same phrasing is found in Proverbs 30:4, "Who has ascended into heaven, or descended? Who has gathered the wind in His fists? Who has bound the waters in a garment? Who has established all the ends of the earth? What is His name, and what is His Son's name, if you know." The answer to the questions is not a man. Only God could do these things, but notice the hint about the Son of God. By stating this using phrasing similar to Proverbs 30:4 Jesus is getting Nicodemus to consider that he might just be addressing the Son of God.