John 6:62

If ye shall see the Son of man ascend - Ye need not be stumbled at what I say concerning eating my flesh and drinking my blood, for ye shall soon have the fullest proof that this is figuratively spoken, for I shall ascend with the same body with which I shall arise from the dead; therefore my flesh and blood, far from being eaten by men, shall not even be found among them.

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.

6:62 What if ye shall see the Son of man ascend where he was before? - How much more incredible will it then appear to you, that he should give you his flesh to eat?