Thomas Aquinas, Compendium of Theology

According to the Apostle, the exaltation of Christ was the reward of His humiliation. Therefore a twofold exaltation had to correspond to His twofold humiliation.

Christ had humbled Himself, first, by suffering death in the passible flesh He had assumed; secondly, He had undergone humiliation with reference to place, when His body was laid in the sepulcher and His soul descended into hell. The exaltation of the resurrection, in which He returned from death to immortal life, corresponds to the first humiliation. And the exaltation of the ascension corresponds to the second humiliation. Hence the Apostle says, in Ephesians 4:10: "He that descended is the same also that ascended above all the heavens."

However, as it is narrated of the Son of God that He was born, suffered and was buried, and rose again, not in His divine nature but in His human nature, so also, we are told, He ascended into heaven, not in His divine nature but in His human nature. In His divine nature He had never left heaven, as He is always present everywhere. He indicates this Himself when He says: "No man hath ascended into heaven but He that descended from heaven, the Son of man who is in heaven" (John 3:13). By this we are given to understand that He came down from heaven by assuming an earthly nature, yet in such a way that He continued to remain in heaven. The same consideration leads us to conclude that Christ alone has gone up to heaven by His own power. By reason of His origin, that abode belonged by right to Him who had come down from heaven. Other men cannot ascend of themselves, but are taken up by the power of Christ, whose members they have been made.

As ascent into heaven befits the Son of God according to His human nature, so something else is added that becomes Him according to His divine nature, namely, that He should sit at the right hand of His Father. In this connection we are not to think of a literal right hand or a bodily sitting. Since the right side of an animal is the stronger, this expression gives us to understand that the Son is seated with the Father as being in no way inferior to Him according to the divine nature, but on a par with Him in all things. Yet this same prerogative may be ascribed to the Son of God in His human nature, thus enabling us to perceive that in His divine nature the Son is in the Father Himself according to unity of essence, and that together with the Father He possesses a single kingly throne, that is, an identical power. Since other persons ordinarily sit near kings, namely, ministers to whom kings assign a share in governing power, and since the one whom the king places at his right hand is judged to be the most powerful man in the kingdom, the Son of God is rightly said to sit at the Father's right hand even according to His human nature, as being exalted in rank above every creature of the heavenly kingdom.

In both senses, therefore, Christ properly sits at the right hand of God. And so the Apostle asks, in Hebrew 1:13: "To which of the angels said He at any time: Sit on My right hand?"

We profess our faith in this ascension of Christ when we say in the Creed: "He ascended into heaven, sitteth at the right hand of God the Father."