21 And furthermore, to make all self-deception unlawful, that Wisdom, which you have yourself confessed to be Christ, shall confront you with the words, When He was establishing the fountains under the heaven, when He was making strong the foundations of the earth, I was with Him, setting them in order. It was I, over Whom He rejoiced. Moreover, I was daily rejoicing in His sight, all the while that He was rejoicing in the world that He had made, and in the sons of men. Every difficulty is removed; error itself must recognise the truth.
There is with God Wisdom, begotten before the worlds; and not only present with Him, but setting in order, for She was with Him, setting them in order. Mark this work of setting in order, or arranging. The Father, by His commands, is the Cause; the Son, by His execution of the things commanded, sets in order. The distinction between the Persons is marked by the work assigned to Each. When it says Let us make, creation is identified with the word of command; but when it is written, I was with Him, setting them in order, God reveals that He did not do the work in isolation.
For He was rejoicing before Him, Who, He tells us, rejoiced in return; Moreover, I was daily rejoicing in His sight, all the while that He was rejoicing in the world that He had made, and in the sons of men. Wisdom has taught us the reason of Her joy. She rejoiced because of the joy of the Father, Who rejoices over the completion of the world and over the sons of men. For it is written, And God saw that they were good. She rejoices that God is well pleased with His work, which has been made through Her, at His command.
She avows that Her joy results from the Father's gladness over the finished world and over the sons of men; over the sons of men, because in the one man Adam the whole human race had begun its course. Thus in the creation of the world there is no mere soliloquy of an isolated Father; His Wisdom is His partner in the work, and rejoices with Him when their conjoint labour ends.
Source: On the Trinity (New Advent)