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 (Question [16], Articles [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]).