This is a very important message for those who do not know God., page-125

@Sandash

Re whether John 3:13 supports a pre-existing Jesus.

John 3:12 - If I have told you earthly things, and ye believe not, how shall ye believe, if I tell you of heavenly things?

Jesus in the next verse goes on to give, explain a heavenly thing, which he expects they have no chance of believing.
This heavenly thing, is an event yet to happen, prophesy.

3:13 - And no man hath ascended up to heaven, but he that came down from heaven, even the Son of man which is in heaven.

This is in reference to the man Jesus, in the here and now of the times.

Confirming it is a future event -> "Son of man which is in heaven". = He was not in heaven when speaking to them.

The future event unfolds, Jesus dies and has risen;
John 20:17 - Jesus said to her, “Do not cling to Me, for I have not yet ascended to My Father; but go to My brethren and say to them, ‘I am ascending to My Father and your Father, and to My God and your God.’ ”

Hence, he ascend up to heaven for a short space of time and comes back down for 40 days and goes back = "Son of man which is in heaven".
So this is the context of -> "but he that came down from heaven", after -> "Jesus said to her, “Do not cling to Me, for I have not yet ascended to My Father;"

Jesus then goes on to elaborate, expand on what he means in 3:13, a future event.
3:14 - And as Moses lifted up the serpent in the wilderness, 'even so' must the Son of man be lifted up: (an event yet to happen).