This verse is found in a context dealing with the question of Nicodemus which he gave in answer to Jesus’ statement about the need to be born again.
3:9-10. Nicodemus was asking just how such spiritual transformation takes place. Jesus answered that Nicodemus, as the teacher of Israel ought to know. The Old Testament prophets spoke of the new Age with its working of the Spirit (Isa. 32:15; Ezek. 36:25-27; Joel 2:28-29). So the nation’s outstanding teacher ought to understand how God by His sovereign grace can give someone a new heart (1 Sam. 10:6; Jer. 31:33).
3:11. But Nicodemus was not only ignorant of this spiritual realm of which Jesus spoke, but he represented the nation’s unbelief and lack of knowledge. Jesus, like the prophets, spoke to the nation about divine themes but the Jews rejected His witness.
3:12. Since Nicodemus could not grasp the basic teaching of regeneration which Jesus presented in earthly analogies, how could he understand and believe the more abstract heavenly matters such as the Trinity, the Incarnation, and Jesus’ coming glorification?
3:13. In this context, the point of verse 13 is that no one has ever gone into heaven and then come back to earth with the ability to give clear teaching about divine matters. The one exception is Jesus who is the Son of Man (cf. 1:50-51; Dan. 7:13; Matt. 26:64). He is the “Ladder” between heaven and earth with access to both realms (see John 3:31 and 6:38). He “descended” in the Incarnation and “ascended” in the Ascension. He also was in heaven before the Incarnation, and therefore knows the divine mysteries. Elijah was taken into heaven, but he has not returned.