Here are three different views:
But the passage (John 6) as a whole certainly reflects also the crisis (present for all Christian centuries) of John’s own community, the difficulty involved in accepting Jesus as the sacramental bread of life. To this difficulty will be added the scandal of the ascent of the Son of Man “to where he was before” (v. 62). The first step of that ascent will be Jesus’ elevation onto a cross on top of a hill.
The elliptic question: ‘If then you see the Son of Man ascending where he was before …?’ is intended, not as Maldonatus thought, to increase the scandal, but to rectify what was simply a cannibalistic interpretation. The ascension will perhaps surprise the recalcitrants more, but it will eliminate their chief difficulty about eating the flesh of One who in celestial glory takes his place where he was from eternity.
Some suggest that if the disciples were to see Jesus ascend to where he was before, their difficulties would be greater. Others say that the use of anabainein refers to the cross, and similarly suggest that the offense would be greater. A third group claims that if they were to see him ascend where he was before their problems would be diminished, as they would know that he had authority to make such statements. The above interpretation is a development of this third option, linking it to Jewish speculation that surrounded the ascensions of the great revealers of Israel (cf. notes on 1:18 and 3:13). If the disciples were to see Jesus ascend—just as they believed the greater revealers from Israel’s sacred history, and especially Moses, had ascended—then would they be prepared to accept his “hard word”?