Within Christianity, there are a great many mysteries about God that we cannot explain, but have to accept through our faith. Our Gospel today contains one of those mysteries, how Our Lord can be both fully human and fully divine.
We see a scene in the Gospel which should be very familiar to each of us. After explaining to His Apostles that He is to suffer and die on the Cross, Jesus takes three of them, Peter, James, and John, up Mount Tabor in the region of Galilee. There, he is changed, and becomes as bright as the sun. At the same time, two of the greatest figures in Jewish history appear and begin to converse with him. Elijah, considered one of the greatest of the prophets, and Moses, who gave the law which dictated Jewish practices both in their daily lives and in their religious observances.
Through this transfiguration, this change in Our Lord's appearance, Jesus showed His Apostles that He was more than just a wise human teacher. His man, whom the Apostles spent most of their time following, was also the Son of God! He was not only a Son by adoption, but fully the Son of God by nature. Jesus, this rabbi from Galilee is both fully human and fully divine.
As far as mysteries go with in Christianity, this is one of the most difficult to understand. How can one person, Jesus, be both fully human and fully divine? Many heresies have surrounded what has been revealed to us by God about the relationship of the divinity and humanity of Jesus. Was he two persons, one human and one divine? How about half-human, half-divine? The short answer is no to both questions. No, he wasn't two persons and he wasn't half-human, half-divine.
In a way of explaining how this works, there is a ritual which priests perform at every Mass, without exception. After pouring the wine into the chalice, the priest takes water and pours just a little drop into one of the chalices saying, “By the mystery of this water and wine, may we come to share in the divinity of Christ who humbled himself to share in our humanity.” From that point, the water can no longer be removed from the chalice, as it has completely mixed in with the wine. Even boiling will not separate just that drop from the rest of the wine, as it would boil off with the water that is naturally within the wine. You still will not be able to remove just that drop of water.
In the same way, the human nature of Jesus is completely inseparable from His divine nature. Within the Creed that we profess at Mass, we say that He is “one in being with the Father”, and that He “came down from Heaven and became man”. He was not an apparition that appeared to be human but was fully divine, as one classic heresy put it. He also was not a created being that was adopted by God from the beginning. The Council of Chalcedon declared in 451 that Jesus was fully human and fully divine “without confusion and without change, without division and without separation”. To put this in simpler language, Jesus wasn't schizophrenic. He wasn't a human who was adopted and made fully divine by God. He wasn't made up of two persons, one human, the other divine. He also wasn't a half-human, half-divine hybrid.
This might seem like making extremely fine distinctions, but it has grave consequences when considering Our Lord's Sacrifice on the Cross. Abraham was willing to sacrifice Isaac in obedience to God's promise, but this sacrifice would only affect Abraham and no one else. By Jesus being fully divine and fully human, He was able to take on the sins and nature of all humanity. A classic formulation in theology is, “That which is not assumed is not redeemed.” If Our Lord would not have taken on the fullness of humanity, He would not be able to redeem all of humanity and its sins.
This was just the tip of the iceberg of what theologians have come up with surrounding Jesus being fully human and fully divine. Even with all that these theologians have learned about Our Lord, we still are no closer to understanding what this truly means than the Apostles were when standing on Mount Tabor watching the transfigured Jesus speaking to Elijah and Moses. We just have to take this mystery which has been revealed to us as a matter of faith.