Jesus often refers to Himself as the Son of Man. It happens so often in each and every one of the Gospel accounts that it almost becomes cliche and, because of its pure saturation in Scripture, I would wager it often gets overlooked. The term Son of Man is in fact incredibly loaded and carries with it both a simple, mortal account and a complex, celestial account that are both equally rooted in the Old Testament.
There are many references to Son of Man throughout the Old Testament. However there are two contexts in which the title or phrase is used. In one context the phrase means mortal, or “mere man” (Num 23:1; Job 25:6; Ps 8:4; Sir 17:30). In this sense, when Jesus calls Himself the Son of Man, He may very well be calling Himself a human, the son of another human and therefore a human Himself. The problem with just that interpretation is that Jesus often linked His use of the term Son of Man with powerful divine prerogatives granted to Him. As the Son of Man, Jesus claims authority to forgive sins (Mk 2:10), suspend the Sabbath (Mk 2:28), judge men for their deeds (Jn 5:27) and even claims to have come down to Earth from Heaven (Jn 3:31). These are important claims to authority that are purposefully linked to Jesus being the Son of Man, and the reason is present within the Book of Daniel.
In chapter 7, the prophet Daniel spends a great deal of time and words imparting a jarring vision that he had while sleeping. In his dream, a series of beasts walk out of the sea in succession. These monsters, Daniel tells us, represent the pagan empires who are hostile to Israel. The most terrible and largest of these beasts is the fourth, who rises from the sea and begins to trample on the “saints of the Most High” (Dan 7:25). The vision suddenly cuts to a Heavenly court. A being “like a son of man” is escorted into the courtroom on the clouds of Heaven (Dan 7:13). During the trial, the son of man is deemed worthy and is given a kingdom that is unmatched in size and prestige to any in history. In addition to the coronation, the fourth beast is condemned by the court and its dominions are handed over to the son of man and the Saints of God (Dan 7:26-27).
In this version of the Son of Man, we see a being in the likeness of a human being granted authority over all of creation, including the dominions of the fourth beast. This is where Jesus anchors His claims to divine authority and it is why He is clear in linking the Son of Man with this divine majesty and power.
What does this have to do with the Transfiguration?
Now about eight days after these sayings he took with him Peter and John and James, and went up on the mountain to pray. And as he was praying, the appearance of his countenance was altered, and his clothing became dazzling white. (Luke 9:28-29)
When Jesus takes the three up the mountain and is transfigured before them, His appearance and His being do not change outside of the confines of his manhood. Jesus is the Son of Man– God and human— in the same way that the term Son of Man has a meaning that is both humble and divinely profound. And when Jesus is transfigured to reveal His Glory and His favour with God, He remains human. We are told of how His being was altered but not so much that He was no longer recognizable to His friends. And neither were Moses or Elijah who came to join Jesus in His Glory. Jesus glorifies the human body in the same manner as He glorifies His Father as the Son of Man.
The Son of Man is a great mystery contained within the Catholic deposit of faith. But just because it is mystery does not mean that it lacks meaning and purpose sustained throughout the entire Gospels and Scripture.