"No one has ever gone into heaven except the one who came from heaven—the Son of Man" (Jn 3:13). What about Elijah and Enoch? I'm confused.Look at the context:
Jn 3:11-15 Truly, truly, I say to you, we speak of what we know, and bear witness to what we have seen; but you do not receive our testimony. 12 If I have told you earthly things and you do not believe, how can you believe if I tell you heavenly things? 13 No one has ascended into heaven but he who descended from heaven, the Son of man. 14 And as Moses lifted up the serpent in the wilderness, so must the Son of man be lifted up, 15 that whoever believes in him may have eternal life."
The context reveals that the main reason Jesus said this was to affirm the authority of his teaching. These "heavenly things" that Jesus is imparting to Nicodemus do not come from just any man, but from he who himself came from heaven and who will return there once he is lifted up like the serpent in the wilderness.
Now, metaphorically speaking, "ascending into heaven" may have the meaning of "believing heavenly things." After all, right after Jesus said that Nicodemus was not open to receiving heavenly things, Jesus says that no one has ascended to heaven. When we are in the Spirit, grasping spiritual things, our mind, in a sense, ascends to heaven. But, when we are not in the Spirit, our mind cannot ascend in this way.
Literally speaking, it means just what it says: no one has ascended into heaven. This too is true, even in the case of Enoch and Elijah, b/c the gates of heaven were closed by the fall of Adam (as symbolized in Gen 3:22-24)....that is, until Jesus conquered death and Hades and led with him into heaven all the righteous souls who came before him (cf. Eph 4:8-10). When the OT speaks of heaven, they do not mean "heaven" as we understand it. The place that we know simply was not available to anyone, no matter how righteous, until Jesus unlocked the gates of Hades (cf. Rev 1:18). He alone holds the key.
I hope that helps.