We Christians believe that God is one because the Bible teaches this. To Moses, first of all, God discloses his name to be Yahweh (Exod 3:14), and he insists that he alone is God and he alone is to be worshipped (Exod 20:2–6; cf. Isa 42:8; 44:6; Zech 14:9). The belief that God is one is underlined in the so-called Shema, the Jewish confession that says, “The Lord our God, Yahweh, is one” (Deut 6:4). However, it is made plain in the Old Testament that Yahweh is not “one” in any abstract, monistic sense. The word translated into English “one” in the Shema is the Hebrew word echad. It can be used to speak of the unity of husband and wife. In the New Testament, the belief that God is one continues to be affirmed (Mark 12:29; Rom 3:30; 1 Cor 8:4–6; Eph 4:6; 1 Tim 2:5; Jas 2:19).

But we Christians also believe that God is three “persons,” because he reveals himself as the Father, the Son, and the Holy Spirit. The word “person” has its limitations because the divine three are more profoundly “one” than any human union, and because this word refers primarily to creatures and only analogically or metaphorically to the divine three. Nevertheless, the divine three are rightly called “persons” because they are revealed as persons: as a Father and a Son who love, relate, speak, and act, and as a Spirit who does likewise. Indeed, in the New Testament, the three divine persons are so clearly depicted as persons that, if it were not for the revelation that God is one, we Christians would be tritheists.

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To conclude these comments on the terms used in Trinitarian orthodoxy of what is one in God (ousia, physis, essentia, substantia), it is important to add that they are not the cause, origin, or source of anything. These synonymous terms in Trinitarian discourse speak of what is common to and unites the divine persons — what makes them the one God.

In concluding my summary of the orthodox doctrine of the Trinity, I pointed out that the Nicene and Athanasian creeds and the Reformation and post-Reformation confessions prescribe what has been concluded is the teaching of Scripture on the doctrine of the Trinity. The Athanasian Creed gives the fullest account, and, to conclude, I note what it says. …

… The Athanasian Creed begins,

We worship one God in Trinity, and Trinity in unity, neither confounding the persons nor dividing the substance. For there is one person of the Father, another of the Son, and another of the Holy Spirit. But the Godhead of the Father, of the Son, and of the Holy Spirit is all one: the glory equal, the majesty coeternal. Such as the Father is, such is the Son: and such is the Holy Spirit.

After reiterating that God is one and yet three equal persons in the clauses following, this creed then declares that the human mind cannot comprehend the divine persons.

The Father is incomprehensible, the Son is incomprehensible: and the Holy Spirit is incomprehensible.

This is a basic tenet of orthodoxy. We only know what God reveals of himself, and even what is revealed cannot be fully grasped by fallen human beings. After this, we have two clauses specifically denying that the Son is less than the Father in authority:

So likewise the Father is almighty, the Son almighty, and the Holy Spirit almighty. And yet there are not three almighties, but one almighty.

So likewise the Father is Lord, the Son is Lord, and the Holy Spirit is Lord. And yet not three Lords, but one Lord.

Then comes a clause that grounds divine threefold differentiation in differing origination. The Father is “not begotten,” the Son is “not created but begotten,” and the Spirit proceeds from the Father and the Son.

Finally, to sum up what is to be confessed in worshipping the Trinity, this creed says,

In this Trinity none is before or after another: none is greater or less than another; but the whole three persons are coeternal together and coequal.

These words absolutely exclude hierarchical ordering of any kind in divine life. The wording could not be more explicit.