Augustine of Hippo
Catena Aurea · Johannesevangelium 6
Many therefore of his disciples, when they had heard this, said, This is an hard saying; who can hear it? When Jesus knew in himself that his disciples murmured at it, he said unto them, Doth this offend you? What and if ye shall see the Son of man ascend up where he was before? It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life. But there are some of you that believe not. For Jesus knew from the beginning who they were that believed not, and who should betray him. And he said, Therefore said I unto you, that no man can come unto me, except it were given unto him of my Father. From that time many of his disciples went back, and walked no more with him. Then said Jesus unto the twelve, Will ye also go away? Then Simon Peter answered him, Lord, to whom shall we go? thou hast the words of eternal life. And we believe and are sure that thou art that Christ, the Son of the living God. Jesus answered them, Have not I chosen you twelve, and one of you is a devil? He spake of Judas Iscariot the son of Simon: for he it was that should betray him, being one of the twelve.
Or, these words are an answer to their mistake. They supposed that He was going to distribute His body in bits: whereas He tells them now, that He should ascend to heaven whole and entire: What and if ye shall see the Son of man ascend up where He was before? ye will then see that He does not distribute His body in the way ye think. Again; Christ became the Son of man, of the Virgin Mary here upon earth, and took flesh upon Him: He says then, What and if ye shall see the Son of man ascend up where He was before? to let us know that Christ, God and man, is one person, not two; and the object of one faith, not a quaternity, but a Trinity. He was the Son of man in heaven, as He was Son of God upon earth; the Son of God upon earth by assumption of the flesh, the Son of man in heaven, by the unity of the person.