Divine simplicity means that God is not composed of different parts, but utterly whole and indivisible. Its opposite is not “complex” but “composite.” To affirm divine simplicity is to affirm that each of God’s attributes is identical with his essence: God is not merely loving and righteous and holy, but Love and Righteousness and Holiness. Whatever God is, He wholly is.
I used to really struggle to understand how the doctrine of God’s simplicity fits together with the doctrine of the Trinity. After all, if God has no parts, how can we say that He exists in three Persons? How is it not inconsistent to affirm both that God has distinct relations within Himself and that He is simple and indivisible? Lately I have come across some helpful treatments of this problem in some older theologians.
First, Anselm in chapter 23 of Proslogion: “you are so simple that there cannot be born of You any other than what You are. This itself is the Love, one and common to You and Your Son, that is the Holy Spirit, proceeding from both…. Whatever each is singly, that the whole Trinity is altogether, Father, Son, and Holy Spirit; since each singly is not other than the supremely simple unity and the supremely unified simplicity which can be neither multiplied nor differentiated.”
Second, Basil the Great (On the Holy Spirit, 18.23): “how does one and one not equal two Gods? Because we speak of the emperor, and the emperor’s image – but not two emperors…. Since the divine nature is not composed of parts, union of the persons is accomplished by partaking of the whole.”
And finally, Bavinck (Reformed Dogmatics, vol. 2, 149): “nor is simplicity inconsistent with the doctrine of the Trinity, for the term simple is not an antonym of ‘twofold’ or ‘threefold’ but of ‘composite.’ God is not composed of three persons, nor is each person composed of the being and attributes of that person, but the one uncompounded (simple) being of God exists in three persons.”
These passages have helped me to see not only are divine simplicity and trinity compatible, but they stand in the closest possible relation and inform one another. The crucial insight that helped remove any sense of inconsistency was seeing that there is a difference between divisions in the Godhead and distinctions in the Godhead. The Father, Son, and Spirit are not divisions, or composite parts, in the Godhead – they don’t combine together to add up to God. Its not as though the Father equals 33.3% of God, the Son another 33.3%, and the Spirit the remaining 33.3%. Rather, the three Persons are distinctions, or relations, in the one God, each one being fully God Himself. As Anselm puts it: “whatever each is singly, that the whole Trinity is altogether.” And how is this? Because of divine simplicity. If God were divisible, we might be tempted to veer off into tritheism. But because God has no divisions or parts, the Son who is begotten by the Father is not other than the Father, and the Spirit who is generated from both is not other than the Father or the Son.
In other words, not only are simplicity and trinity compatible, but they interpret and protect one another.