John 6:61-62
“When Jesus knew in Himself that His disciples murmured at it,” (for this is an attribute of His Godhead to bring secret things to light,) “He said unto them, Does this offend you? What and if you shall see the Son of Man ascend up where He was before?”
This also He does in the case of Nathanael, saying, “Because I said unto you, I saw you under the fig-tree, do you believe? You shall see greater things than these.” And to Nicodemus, “No man has ascended up to heaven but the Son of man which is in heaven.” What then, does He add difficulties to difficulties? No, (that be far from Him,) but by the greatness of the doctrines, and the number of them, He desires to bring them over. For if one had said simply, “I have come down from heaven,” and added nothing more, he would have been the more likely to offend them; but He who said, “My body is the life of the world”; He who said, “As the living Father has sent Me, so I live by the Father”; and who said, “I have come down from heaven,” solves the difficulty. For the man who utters any one great thing concerning himself may perhaps be suspected of feigning, but he who connects together so many one after another removes all suspicion. All that He does and says is intended to lead them away from the thought, that Joseph was His father. And it was not with a wish to strengthen, but rather to do away that stumbling-block, that He said this. For whosoever deemed that He was Joseph's son could not receive His sayings, while one that was persuaded that He had come down from heaven, and would ascend there, might more easily give heed to His words: at the same time He brings forward also another explanation, saying,
Source: Homilies on the Gospel of John (New Advent)