27 But remember also what I have often said concerning the Son's sitting at the right hand of the Father; because of the next sentence in the Creed, which says, “and ascended into Heaven, and sat down at the right hand of the Father.” Let us not curiously pry into what is properly meant by the throne; for it is incomprehensible: but neither let us endure those who falsely say, that it was after His Cross and Resurrection and Ascension into heaven, that the Son began to sit on the right hand of the Father. For the Son gained not His throne by advancement; but throughout His being (and His being is by an eternal generation) He also sits together with the Father. And this throne the Prophet Esaias having beheld before the incarnate coming of the Saviour, says, I saw the Lord sitting on a throne, high and lifted up , and the rest. For the Father no man has seen at any time , and He who then appeared to the Prophet was the Son. The Psalmist also says, Your throne is prepared of old; You are from everlasting . Though then the testimonies on this point are many, yet because of the lateness of the time, we will content ourselves even with these.
Source: Catechetical Lectures (New Advent)