36 Now the Apostle, maintaining the true sense of the Lord's saying, I and the Father are one, while He asserts that Both are One, signifies that Both are One not after the manner of the soleness of a single being, but in the unity of the Spirit; for one God the Father and one Christ the Lord, since Each is both Lord and God, do not yet admit in our creed either two Gods or two Lords. So then Each is one, and though one, neither is sole. We shall not be able to express the mystery of the faith except in the words of the Apostle.
For there is one God and one Lord, and the fact that there is one God and one Lord proves that there is at once Lordship in God, and Godhead in the Lord. You can not maintain a union of person, so making God single; nor yet can you divide the Spirit, so preventing the Two from being One. Nor in the one God and one Lord will you be able to separate the power, so that He Who is Lord should not also be God, and He Who is God should not also be Lord. For the Apostle in the enunciation of the Names has taken care not to preach either two Gods or two Lords.
And for this reason he has employed such a method of teaching as in the one Lord Christ to set forth also one God, and in the one God the Father to set forth also one Lord. And, not to misguide us into the blasphemy that God is solitary, which would destroy the birth of the Only-begotten God, he has confessed both Father and Christ.
Source: On the Trinity (New Advent)