What Jesus is telling them you can't totally understand heavenly things without the Spirit, and that's what Paul is talking about at 1 Cor 2:13, but there are some of you who do not believe. When you put the facts together and you have to learn as you go along, when you read the Gospel of John, you will see the meaning of John 20:28, when Thomas says to the Risen Jesus, my Lord and my God. You just don't go there and forget about everything that you were taught up to that point, you're supposed to learn as you go through the Gospel of John that's the point, that's why John is telling you all this spiritual things so that you can learn as you go on. Thomas saw Jesus with God in Him. We see that the spirit was descending out of heaven upon Jesus Joh 1:32, Joh 1:51, When you are born of the Spirit that's where your new life originates, from heaven, you are from heaven. As a child of God, this is the way you should think about of yourself if you're born again. Your new life originated with God the Father in heaven He's our Father and that's where your new life is, born of God, born from above. The Spirit descending out of heaven. When Jesus talks about ascending to where He was before He tells his disciples it is the spirit that gives life, the spirit that descended out of heaven, That's why he can talk about the Son of Man ascending to where He was before, The spirit comes from heaven and it will return to heaven and He's not talking about a different part of himself, He's talking about himself as a human being a fleshly human being.
Remember Jesus's body did NOT descend from heaven, the body was formed in Mary. What descended from heaven? YHWH's Holy Spirit descended from heaven. Even if you think it happened at his conception, in both cases, it was YHWH's Holy Spirit that descended from heaven, not a fleshy body that did. Yet he said his body descended from heaven. In the same way our flesh did not descend from heaven, but when we are born again, the "word of God" (YHWH's Holy Spirit) becomes our flesh, and we can then say we've descended from heaven. How else did you expect to get into the heavenly kingdom?
In John 3:13 Jesus emphasizing the words “son of man” “human being”, see also John 6 about his flesh, there's a spiritual side to that flesh that people couldn't see. Three disciples did see it on the mountain when He took him up and was transfigured. The spirit gives life and that's why these verses fit together as they do in John 6:62. The spirit was up in heaven before, and that is where Jesus life originates with God in heaven, and so are we who are born again. I realize it's pretty safe to assume that this will create lots of questions from people because people tend to have these preconceptions. It's kind of like if you go to university and you've never taken calculus before but you've taken all kinds of algebra and all other kind of stuff and your thinking pattern is mathematics. And then on your first week you get calculus thrown at you and you're trying to apply the principles of algebra and all that kind of stuff to this new thing called calculus. If you do that you're gonna have a big trouble because calculus is a whole different deal and that's sort of what's probably going to happen here people are gonna get locked into their preconceptions and not just accept the Scriptures and they wont let the spirit lead them. Even after you show them how this fits and works and that everything works together perfectly in John'ss gospel. God bless.