Since the mid-second century, Christian theologians have described the substance of God as simple. At the most basic level they have meant by this term that God is incomposite being. He is not composed of parts nor can be divided into parts. He is most certainly not constituted of corporeal members, but neither is he made up of distinct and separable qualities or properties. The transcendent deity is not a conglomeration. Andrew Radde-Gallwitz elaborates:
To say God is simple is to provide a sort of second-order rule for speaking about God. At the most basic, affirming divine simplicity means that if one says ‘God is just” and “God is merciful” one does not view God’s justice and mercy as parts of God. But, additionally, it means that one should not take these attributes as contradicting one another–since only complex beings can have contradictory properties at the same time. If one claims divine simplicity, one cannot hold that there is any tension or struggle in God between justice and mercy. Rather, one must articulate the sense in which God is both, reject one or the other, or assign the contradictory attributes to distinct ‘levels’ of divine reality–the latter was at any rate an option up until the Cappadocians. (Basil of Caesarea, Gregory of Nyssa, and the Transformation of Divine Simplicity, p. 6)
Eventually the notion of divine simplicity, in the hands of some theologians, evolved into the identity thesis: the divine substance is metaphysically identical to the divine attributes. Philosopher Alvin Plantinga summarizes the identity thesis in these words: “God does indeed have a nature; but he is identical with it. … We cannot distinguish [God] from his nature or his nature from his existence, or his existence from his other properties; he is the very same thing as his nature, existence, goodness, wisdom, power and the like” (quoted in Radde-Gallwitz, p. 5 n. 8).
Underlying the debate between Eunomius of Cyzicus and St Basil of Caesarea are two conflicting understandings of the divine simplicity. Eunomius is a representative of the identity thesis. Basil also affirms divine simplicity, but when confronted with the Eunomian version of divine simplicity and the way it was employed by the heresiarch to deny important gospel claims, he realized he had to revise the doctrine to render it compatible with and transparent to the apostolic revelation.
As we have seen, Eunomius claims that God has revealed his divine name–“unbegotten.” This name defines the divine substance. To know this name is to be given a sound and exact knowledge of God. It is to know God as he knows himself. On the basis of Eunomius’s understanding of the divine simplicity, he deduces that all names and titles that refer to the divine substance must be synonymous with “unbegotten.” One might think that if we call God incorruptible, this must mean something different than when we call God invisible; yet Eunomius believes that these words, and all other words with which we speak of the being of God, must mean unbegotten, because “unbegotten” properly names God. We may call this the synonymity principle. “Every word used to signify the essence of the Father is equivalent in force of meaning to ‘the Ingenerate’ because the Father is without parts and uncomposed” (Apol. 19). At first blush, this sounds a bit silly, yet it appears to logically follow from Eunomius’s conviction that we may comprehend the simple divine substance. Ingenerateness is identical to the essence of God; hence all other names that describe God must ultimately signify ingenerateness.
St Basil rightly saw that the Eunomian philosophy represented a radical attack on Christian faith and practice. If Eunomius is right, then Jesus cannot be fully divine, and thus a proper object of worship, because he is begotten and therefore cannot share in the being of the unbegotten God. If Eunomius is right, then the multiplicity of names, terms, and titles that are employed in the Holy Scripture to speak of the living God do not mean what they seem to mean.
Eunomius was willing to accept these consequences because, in his view, the denial of “unbegotten” as the true name of the divine substance can only mean the total absence of any knowledge of divinity. “For Eunomius,” Radde-Gallwitz writes, “the doctrine of simplicity functioned to ensure the objective nature of the knowledge of God. Because God has no nonessential properties, in so far as we know God, we know God’s very essence” (p. 112).
In the mid-370s Bishop Amphilochius would write Basil to ask his advice. Apparently he and his congregations were confronted by Eunomian adversaries who charged that because they denied the possibility of comprehending the divine substance they did not know the God whom they worshipped. “God is simple,” the Eunomians confessed, “and everything one might enumerate as knowable about him belongs to his essence” (Epistle 234). If a person cannot precisely characterize the substance of God, then he does not know God at all, and the God he worships is a stranger, no better than the unknown deity of the Athenians (Acts 17). This was the challenge faced by St Basil. Something had to give. Either Basil would have to accept the position of Eunomius that we can define the essence of God or adopt an extreme apophaticism or reconfigure the doctrine of divine simplicity.