15 Wherefore, keep not silent, O Lord, concerning the resurrection of the flesh; lest men believe it not, and we continue reasoners, not preachers. But “as the Father has life in Himself, even so has He given to the Son to have life in Himself.” Let them that hear, understand; let them believe that they may understand; let them obey that they may live. And that they may not suppose that the resurrection is finished here, let them hear this further: “and has given Him authority to execute judgment also.”
Who has given? The Father. To whom has He given? To the Son; namely, to whom He gave to have life in Himself, to the same has He given authority to execute judgment. “Because He is the Son of man.” For this is the Christ, both Son of God and Son of man. “In the beginning was the Word, and the Word was with God, and the Word was God. This was in the beginning with God.” Behold, how He has given Him to have life in Himself! But because “the Word was made flesh, and dwelt among us,” was made man of the Virgin Mary, He is the Son of man.
What, therefore, has He received as Son of man? Authority to execute judgment. What judgment? That in the end of the world. Then also there will be a resurrection, but a resurrection of bodies. So, then, God raises up souls by Christ, the Son of God; bodies He raises up by the same Christ, the Son of man. “Hath given Him authority.” He should not have this authority did He not receive it; and He should be a man without authority. But the same who is Son of God is also Son of man.
For by adhering to the unity of person, the Son of man with the Son of God is made one person, and the Son of God is the same person which the Son of man is. But what characteristic it has, and wherefore, must be distinguished. The Son of man has soul and body. The Son of God, which is the Word of God, has man, as the soul has body. And just as soul having body does not make two persons, but one man; so the Word, having man, makes not two persons, but one Christ. What is man? A rational soul, having a body. What is Christ? The Word of God, having man. I see of what things I speak, who I the speaker am, and to whom I am speaking.
Source: Tractates on the Gospel of John (New Advent)