9, 10. Unless Father and Son are two in name only, or as parts and so each imperfect, or two gods, they are coessential, one in Godhead, and the Son from the Father.
9 'I and the Father are One.' You say that the two things are one, or that the one has two names, or again that the one is divided into two. Now if the one is divided into two, that which is divided must need be a body, and neither part perfect, for each is a part and not a whole. But if again the one have two names, this is the expedient of Sabellius, who said that Son and Father were the same, and did away with either, the Father when there is a Son, and the Son when there is a Father. But if the two are one, then of necessity they are two, but one according to the Godhead, and according to the Son's coessentiality with the Father, and the Word's being from the Father Himself; so that there are two, because there is Father, and Son, namely the Word; and one because one God. For if not, He would have said, 'I am the Father,' or 'I and the Father am;' but, in fact, in the 'I' He signifies the Son, and in the 'And the Father,' Him who begot Him; and in the 'One' the one Godhead and His coessentiality. For the Same is not, as the Gentiles hold, Wise and Wisdom, or the Same Father and Word; for it were unfit for Him to be His own Father, but the divine teaching knows Father and Son, and Wise and Wisdom, and God and Word; while it ever guards Him indivisible and inseparable and indissoluble in all respects.
10. But if any one, on hearing that the Father and the Son are two, misrepresent us as preaching two Gods (for this is what some feign to themselves, and immediately mock, saying, 'You hold two God.'), we must answer to such, If to acknowledge Father and Son, is to hold two Gods, it instantly follows that to confess but one we must deny the Son and Sabellianise. For if to speak of two is to fall into Gentilism, therefore if we speak of one, we must fall into Sabellianism. But this is not so; perish the thought! But, as when we say that Father and Son are two, we still confess one God, so when we say that there is one God, let us consider Father and Son two, while they are one in the Godhead, and in the Father's Word being indissoluble and indivisible and inseparable from Him. And let the fire and the radiance from it be a similitude of man, which are two in being and in appearance, but one in that its radiance is from it indivisibly.
Source: Four Discourses Against the Arians (New Advent)