Consider this. Paul says: “One God, the Father from whom are all things, and one Lord, Jesus Christ, through whom are all things.” If the Father is said to be one God, it excludes the Son from being God; if the Son is said to be one Lord, it excludes the Father from being Lord. But surely the Father is not excluded from being Lord because of Paul’s words: “One Lord, Jesus Christ.” Therefore neither is the Son excluded from being God because of the words: “one God, the Father.”
If the Anomoeans should say in reply that the Father is called one God because the Son is God, but not on the same level as the Father, what follows from the premises they set down? They then must say—for we would not say it—that the Son is called one Lord because the Father is Lord, but not on the same level as the Son. But if to say this is impious, neither would the first statement have any basis in truth. Just as the expression “one Lord” does not exclude the Father from being perfect Lord nor does this title bestow Lordship on the Son alone, so the expressions “one God” does not exclude the Son from being true and genuine God, nor does this title show that Godhead belongs only to the Father.
That the Son is God and, while still remaining the Son, is God on the same level as the Father becomes clear from the very addition of the word “Father.” If this name of God belonged only to the Father and if it could not designate for us another personal reality [hypostasis] but only that first and unbegotten personal reality, inasmuch as the name “God” can belong to and denote only that personal reality, the addition of the name “Father” would serve no purpose.
If the additional designation of “Father” were not meaningful, it would be enough to speak of “one God,” and we would know who was meant. But since the name of “God” is common to the Father and the Son, Paul would not be making it clear of whom he was speaking if he were only to say “one God.” So the addition of “Father” was needed to show that he was speaking of the first and unbegotten personal reality. The name of God would not suffice to show this, since this name is common to him and to the Son.
Some names are common to several; others are proper to one. There are common names to show that the essence is exactly the same; there are proper names to characterize what is proper to the personal realities. The names “Father” and “Son” characterize what is proper to the personal realities; the names “God” and “Lord” show what is common. Therefore, after Paul set down the common name of “one God,” he had to use the proper name so that you might know of whom he was speaking. He did this to prevent us from falling into the madness of Sabellius.