Objection 1. It would seem that it does not belong to Christ as man to sit at the right hand of the Father, because, as Damascene says (De Fide Orth. iv): "What we call the Father's right hand is the glory and honor of the Godhead." But the glory and honor of the Godhead do not belong to Christ as man. Consequently, it seems that Christ as man does not sit at the right hand of the Father.
Objection 2. Further, to sit on the ruler's right hand seems to exclude subjection, because one so sitting seems in a measure to be reigning with him. But Christ as man is "subject unto" the Father, as is said in 1 Corinthians 15:28. Therefore it seems that Christ as man does not sit at the Father's right hand.
Objection 3. Further, on Romans 8:34: "Who is at the right hand of God," the gloss adds: "that is, equal to the Father in that honor, whereby God is the Father: or, on the right hand of the Father, that is, in the mightier gifts of God." And on Hebrews 1:3: "sitteth on the right hand of the majesty on high," the gloss adds, "that is, in equality with the Father over all things, both in place and dignity." But equality with God does not belong to Christ as man; for in this respect Christ Himself says (John 14:28): "The Father is greater than I." Consequently, it appears unseemly for Christ as man to sit on the Father's right hand.
On the contrary, Augustine says (De Symb. ii): "By the expression 'right hand' understand the power which this Man, chosen of God, received, that He might come as judge, who before had come to be judged."
I answer that, As stated above (Article 2), by the expression "right hand" is understood either the glory of His Godhead, or His eternal beatitude, or His judicial and royal power. Now this preposition "at" signifies a kind of approach to the right hand; thus denoting something in common, and yet with a distinction, as already observed (De Symb. ii). And this can be in three ways: first of all, by something common in nature, and a distinction in person; and thus Christ as the Son of God, sits at the right hand of the Father, because He has the same Nature as the Father: hence these things belong to the Son essentially, just as to the Father; and this is to be in equality with the Father. Secondly, according to the grace of union, which, on the contrary, implies distinction of nature, and unity of person. According to this, Christ as man is the Son of God, and consequently sits at the Father's right hand; yet so that the expression "as" does not denote condition of nature, but unity of suppositum, as explained above (16, 10,11). Thirdly, the said approach can be understood according to habitual grace, which is more fully in Christ than in all other creatures, so much so that human nature in Christ is more blessed than all other creatures, and possesses over all other creatures royal and judiciary power.
So, then, if "as" denote condition of nature, then Christ, as God, sits "at the Father's right hand," that is, "in equality with the Father"; but as man, He sits "at the right hand of the Father," that is, "in the Father's mightier gifts beyond all other creatures," that is to say, "in greater beatitude," and "exercising judiciary power." But if "as" denote unity of person, thus again as man, He sits at the Father's right hand "as to equality of honor," inasmuch as with the same honor we venerate the Son of God with His assumed nature, as was said above (Question 25, Article 1).
Reply to Objection 1. Christ's humanity according to the conditions of His nature has not the glory or honor of the Godhead, which it has nevertheless by reason of the Person with whom it is united. Hence Damascene adds in the passage quoted: "In which," that is, in the glory of the Godhead, "the Son of God existing before ages, as God and consubstantial with the Father, sits in His conglorified flesh; for, under one adoration the one hypostasis, together with His flesh, is adored by every creature."
Reply to Objection 2. Christ as man is subject to the Father, if "as" denote the condition of nature: in which respect it does not belong to Him as man to sit at the Father's right hand, by reason of their mutual equality. But it does thus belong to Him to sit at the right hand of the Father, according as is thereby denoted the excellence of beatitude and His judiciary power over every creature.
Reply to Objection 3. It does not belong to Christ's human nature to be in equality with the Father, but only to the Person who assumed it; but it does belong even to the assumed human nature to share in God's mightier gifts, in so far as it implies exaltation above other creatures.
Source: Summa Theologica (New Advent)