For the Christian being a monotheist (belief in one God), and Trinitarian (belief that the one God is three persons) are mutually implicating realities. As you know I have been reading Katherine Sonderegger’s Volume 1 Systematic Theology, Doctrine of God. In it she has many rich insights and laudable things to communicate, but when it comes right down to it I have to finally demur; and the demurring comes because of a fundamental disagreement about how she parses monotheism and Trinitarianism, as if one does not implicate the other and the other does not implicate the one. Let me share from what she has written, and you will see what I mean (maybe you’ll agree with her, but maybe you’ll agree with me).
Note what I said here! Our reading of the priestly-prophetic visio Dei is not principally Trinitarian in character. We are not hearing and seeking out in this witness of ancient Israel a sign and foretaste of Triune Persons. It is not the Father, say, that we see breaking through the cloud and smoke to descend upon Moses and upon the people Israel. Nor do we look for an intimation of the Son in these royal Appearances in the temple. We do not bring forward first and principally the Holy Spirit as personal disclosure in Dame Wisdom or in the maternal brooding over the dark sea at creation’s dawn. The forward press of so much modern theology—the drive to subsume the doctrine of God within the Trinity and the Triune Persons—does not, I believe, properly attest the Unicity of the God of Israel. The Deity and Nature of God is personal: the One God is a Person; we can dare to put it this way. Monotheism is no shame word! At once God is Nature and Person, and the witness of ancient Israel to its Lord is to an Object inalienably Subject, a Subject lowered and handed over to be Object. This oscillation in Israel’s and therefore our religious life before God—now our experience of the I AM—is the gracious condescension of the Lord God to usward, for these are not two, not distinct or segmented, but One, One Mystery, One God.
This is troubling to me on many fronts, primary of which is her rupturing of the One and the Many, the One and the Three; her apparent fear about “the drive to subsume the doctrine of God within the Trinity and the Triune Persons” is problematical to say the least!
She has Karl Barth in her sights (she had just mentioned him and aimed at him in the preceding context leading to the quote I shared), and so she might as well have TF Torrance and Evangelical Calvinists in her sights as well (indeed she does!). There is no need to worry the way she does; she speaks of Monotheism as if, for the Christian, this is somehow, definitionally different than speaking of the Trinity, but it surely isn’t! For the orthodox Christian we know of no other Oneness of God apart from His revealed reality in the Son, Jesus Christ. As such, our understanding of God’s oneness (de Deo uno) is necessarily shaped by His Self-revelation as Father, Son, and Holy Spirit (de Deo trino), the hypostases of the One God. Oneness for the Christian is absolutely subsumed by the Threeness of the persons of the Divine Monarxia (Godhead), not incidentally. We are not Muslims, neither are we Jews, we are not Unitarians we are Trinitarians, and so Sonderegger’s fear is not well founded, not in the Christian tradition; least not in the tradition I affirm.
Let’s end this with TF Torrance, and how he would most likely respond to Sonderegger’s pronouncement about monotheism and the Trinity:
in the Scots Confession as in John Knox’s Genevan Liturgy, the doctrine of the Trinity is not added on to a prior conception of God—there is no other content but the Father, the Son, and the Holy Spirit. There was no separation here between the doctrine of the One God (De Deo Uno), and the doctrine of the triune God (De Deo Trino), which had become Roman orthodoxy through the definitive formalisation of Thomas Aquinas. This trinitarian approach was in line with The Little Catechism which Knox brought back from Geneva for the instruction of children in the Kirk. “I believe in God the Father, and in Jesus Christ his Son and in the Holy Spirit, and look for salvation by no other means.” Within this trinitarian frame the centre of focus in the Confession and Catechism alike is upon Jesus Christ himself, for it is only through him and the Gospel he proclaimed that God’s triune reality is made known, but attention is also given to the Holy Spirit. Here once again we have a different starting point from other Reformation Confessions. Whereas they have a believing anthropocentric starting point, such as in the Heidelberg Catechism, this is quite strongly theocentric and trinitarian. Even in Calvin’s Institute, which follows the fourfold pattern in Peter Lombard’s Sentences, the doctrine of the Trinity is given in the thirteenth chapter within the section on the doctrine of God the Creator. Calvin’s Genevan Catechism, however, understandably followed the order of the Apostles’ Creed. The trinitarian teaching in the Scots Confession was by no means limited to the first article for it is found throughout woven into the doctrinal content of subsequent articles.
Torrance would note that Sonderegger is following the ‘formalisation of Thomas Aquinas,’ indeed she does. Just following the quote I share from KS, she gets into the mode, the method of theological engagement she is using to come to such conclusions; she notes her reliance upon scholastic methodology, and how that implicates our grammar and understanding towards a knowledge of God. As a result she has artificially wrested the Triune Persons from the One God of Israel, and that is too bad!
 Katherine Sonderegger, Systematic Theology: Volume 1, The Doctrine of God (Minneapolis: Fortress Press, 2015), Loc. 6577, 6584 Kindle.
 Torrance, Scottish Theology, 3–4.