The same position we hold respecting the Spirit, who has that unity with the Son which the Son has with the Father. Wherefore let the hypostasis of the Father be discriminated by the appellation of God; but let not the Son be cut off from this appellation, for He is of God. Again, let the person of the Son also be discriminated by the appellation of Lord; only let not God be dissociated from that, for He is Lord as being the Father of the Lord. And as it is proper to the Son to exercise lordship, for He it is that made (all things) by Himself, and now rules the things that were made, while at the same time the Father has a prior possession of that property, inasmuch as He is the Father of Him who is Lord; so we speak of the Trinity as One God, and yet not as if we made the one by a synthesis of three: for the subsistence that is constituted by synthesis is something altogether partitive and imperfect. But just as the designation Father is the expression of originality and generation, so the designation Son is the expression of the image and offspring of the Father.
Hence, if one were to ask how there is but One God, if there is also a God of God, we would reply that that is a term proper to the idea of original causation, so far as the Father is the one First Cause. And if one were also to put the question, how there is but One Lord, if the Father also is Lord, we might answer that again by saying that He is so in so far as He is the Father of the Lord; and this difficulty shall meet us no longer.
Source: A Sectional Confession of Faith (New Advent)