Basil elaborates on the unity of the Father and the Son by invoking the divine Monarchy. Contrary to the assertions of the Anomoians, the Son does not possess a nature different from God simply because he is from God:
Nevertheless, you should not usurp the distinction of the persons for impiety. For even if they are two in number, they are not disjoined in nature. Nor does anyone who says “two” introduce estrangement [between them]. There is one God because there is one Father. But the Son is also God, and there are not two gods because the Son has identity with the Father. For I do not behold one divinity in the Father and another in the Son. Nor is one nature this and the other that. So then, in order to make clear for you the distinctness of the person, count the Father by himself and the Son by himself, but in order to avoid secession into polytheism, confess one substance in both. In this way both Sabellius falls and the Anomoian will be shattered.
But when I say “one substance,” do not think that two are separated from one, but that the Son has come to subsist from the Father, his principle. The Father and Son do not come from one substance that transcends them both. For we do not call them brothers, but confess Father and Son. There is an identity of substance because the Son is from the Father, not made by a command but rather begotten from his nature, not separated from him but the perfect radiance of the Father, who himself remains perfect. (Sab. 3-4)
This is an important passage and deserves careful reading and analysis. The Eastern presentation of trinitarian doctrine is often misunderstood by Westerners (nobody but us Westerners here). Without thinking we immediately jump to the consubstantiality of the divine persons. The words of the Quicunque vult resound in our ears: “So the Father is God; the Son is God; and the Holy Ghost is God.” We hardly notice the creedal sentences that express the divine processions: the Father begets the Son; the Spirit proceeds from the Father.
But Basil, following the witness of Scripture and creed, proposes the Monarchy of the Father as that which explains the equal divinity of Son and Spirit. The story begins with the God of Israel, the One who has spoken the world into existence and has summoned a people to himself. To them he has shared his covenant name, YHWH. In the Incarnation, though, a new revelation of YHWH is given: God is the Father of Jesus, and Jesus is his only begotten Son. Thus Basil’s multiple exhortations to attend to the terms Father and Son and comprehend their meaning. Each term mutually implicates the other. We must “learn from each designation a distinct notion,” says Basil (Sab. 3). He does not retract or deny the biblical assertion of the One Creator whom Jesus identifies as his Father; rather he explains that the Father is Father precisely by generation of the Son. If there were no eternal Son, there would be no eternal Father. God is never alone. When we confess “I believe in one God, the Father Almighty,” we do not thereby exclude Christ Jesus, for God enjoys eternal and mutually defining relationship with the Only-Begotten.
The affirmation of the divinity of the Son does not, insists Basil, result in a form of pagan polytheism. The Scripture teaches us that there is a way for God to generate an hypostasis distinct from himself and yet who shares equally his divine nature. The divine generation, however, must not be understood in temporal terms. It’s not as if there first exists the unbegotten Deity who then subsequently begat the Son. The generation of the Son occurs eternally, timelessly, ineffably within the Godhead. Polytheism is thus avoided. There is only one first principle, the God and Father of the Lord Jesus Christ:
And those of you who have not come around to my position, as much as you have either imperfectly followed what we have said or stood around only hoping to insult us, not seeking to receive any benefit from us but looking to seize on something that we have said, still you say: “He preaches two gods! He proclaims polytheism!” There are not two gods because there are not two fathers. Whoever introduces two first principles preaches two gods. Such is Marcion and anyone similar to him in impiety. Once again, whoever says that the one begotten is different in substance from the one who has begotten him implies that there are two gods, introducing polytheism by maintaining the unlikeness of substance. For if there is one divinity that is unbegotten and one that is begotten, you are preaching polytheism, implying that the unbegotten is contrary to the begotten and clearly positing too that their substances are contrary—that is, if indeed unbegottenness is the substance of the Father and begottenness is the substance of the Son. Thus you imply not only that there are two gods, but also that they are opposed to each other. And the worst of it is that you do not locate their conflict in the will but in a natural dissension which can never be peacefully resolved. (Sab. 4)
Basil thus turns the tables on his accusers. His orthodox presentation of the Trinity avoids polytheism because it asserts (1) one transcendent first principle, namely, God the Father, and (2) the substantial unity of the Father and the Son.