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.

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 ye shall see the son of man,.... Meaning himself then in a state of humiliation, and was taken for a mere man, though the true Messiah, and Son of God:

ascend up where he was before? for Christ was, he existed before his incarnation, and he was in heaven before; not in his human nature, but as the word and Son of God: and he intimates, that when he had done his work, and the will of his Father, for which he came down from heaven, by the assumption of the human nature, he should ascend up thither again; and which would be seen, as it was, by his apostles; and which would prove that he came down from heaven, as he had asserted; see Ephesians 4:9; and that his flesh and blood were not to be eaten in a corporeal sense; in which sense they understood him: and he hereby suggests, that if it was difficult to receive, and hard to be understood, and was surprising and incredible, that he should come down from heaven, as bread, to be eat and fed upon; it would be much more so to them to be told, that he who was in so mean and lowly a form, should ascend up into heaven.

What and if ye shall see (ἐὰν οὐν θεωρῆτε)

The question is marked by an aposiopesis, i.e., a breaking off of the sentence and leaving the hearer to complete it for himself. Literally, if then ye should behold, etc. - the completion would be, would not this still more cause you to stumble?

Ascend (ἀναβαίνοντα)

Rev., properly, renders the participle, ascending.

I speak (λαλῶ)

But the correct reading is λελάληκα, the perfect tense, I have spoken, or I have just spoken, referring to the preceding discourse.

What and if ye shall see the Son of man ascend up where he was before?

6:62 What if ye shall see the Son of man ascend? He points forward to a greater marvel than the one that now staggered before them, the Ascension of the Son of man.

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?

6:60-65 The human nature of Christ had not before been in heaven, but being God and man, that wondrous Person was truly said to have come down from heaven. The Messiah's kingdom was not of this world; and they were to understand by faith, what he had said of a spiritual living upon him, and his fulness. As without the soul of man the flesh is of no value, so without the quickening Spirit of God all forms of religion are dead and worthless. He who made this provision for our souls, alone can teach us these things, and draw us unto Christ, that we may live by faith in him. Let us apply to Christ, thankful that it is declared that every one who is willing to come unto him shall be made welcome.