Albert Barnes Commentary John 6:62

"[What] then if ye should behold the Son of man ascending where he was before?" — John 6:62 (ASV)

What and if... Jesus does not say that those who were then present would see him ascend, but he implies that he would ascend. They had taken offence because he said he came down from heaven. Instead of explaining that away, he proceeds to state another doctrine quite as offensive to them—that he would reascend to heaven. The apostles only were present at his ascension, Acts 1:9. As Jesus was to ascend to heaven, it was clear that he could not have intended literally that they should eat his flesh.