On the Christian Faith (De fide)

Forasmuch as God is One, the Son of God is God, good and true.

33. Yet what think you, who deny the goodness and true Godhead of the Son of God, though it is written that there is no God but One? For although there be gods so-called, would you reckon Christ among them which are called gods, but are not, seeing that eternity is of His Essence, and that beside Him there is none other that is good and true God, forasmuch as God is in Him; while it follows from the very nature of the Father, that after Him there is no other true God, because God is One, neither confounding [the Persons of] the Father and Son, as the Sabellians do, nor, like the Arians, severing the Father and the Son. For the Father and the Son, as Father and Son, are distinct persons, but they admit no division of their Godhead.