Hebrews 5:5

5.Thou art my Son, etc. This passage may seem to be far­fetched; for though Christ was begotten of God the Father, he was not on this account made also a priest. But if we consider the end for which Christ was manifested to the world, it will plainly appear that this character necessarily belongs to him. We must however bear especially in mind what we said on the first chapter; that the begetting of Christ, of which the Psalmist speaks, was a testimony which the Father rendered to him before men. Therefore the mutual relation between the Father and the Son is not what is here intended; but regard is rather had to men to whom he was manifested. Now, what sort of Son did God manifest to us? One indued with no honor, with no power? Nay, one who was to be a Mediator between himself and man; his begetting then included his priesthood. (86)

5.So also Christ, himself he did not exalt to be a high priest, but he who had said to him, “My son art thou, I have this day begotten thee.”

It is the same as though he had said, “Christ did not make himself a high priest but God.” And the reason why he speaks of God as having said “My Son,” etc., seems to be this, — to show that he who made him king (for the reference in Psalms 2:7 is to his appointment as a king) made him also a high priest. And this is confirmed by the next quotation from Psalms 110:1; for in the first verse he is spoken of as a king, and then in verse 4 his priesthood is mentioned. — Ed.