For Thomas F. Torrance the homoousion is key. The homoousion is the patristic development that articulates the reality of the consubstantial relation that inheres between the eternal Logos, and the eternal Father (and the eternal Spirit); i.e. the idea that the Son is eternal God, just as the Father and Spirit are (One-in-Three/Three-in-One). Torrance believes that it is only in this reality that any genuine objective 
knowledge of God can be acquired; the alternative, within the patristic context, and within which the homoousion developed against, would be something like Arianism or Eunomianism where the axiom was: ‘there was a time when the Son was not’ (my paraphrase). Torrance argues (in more developed and elaborate ways than we will touch upon here) that the Arian way, since it only thinks in immanent ways, or from humanistically rationalist ways, thinks from within a closed-in circle wherein it can never escape its own wits, particularly when attempting to think God. Torrance maintains that in order to transcend our ‘wits’ we must be confronted with God in such a way that he enters into our mode of being, and serves as the mediator between God and humanity in such a way that we can think God, not from our thoughts or a center in ourselves, but from his thoughts for us, from a center in Godself who Christ is as the eternal Logos (John 1.1).
The homoousion, while not a ‘biblical’ term (same as the term ‘Trinity’), Torrance argues is a necessary term that naturally arises from the interior logic deposited for us in the Apostolic witness; i.e. the New Testament. This is such a central tenet for Torrance that he believes that real-deep Bible study requires that we go beyond the simple grammatical, syntactical, or literary components found in the text of the New Testament (and Old), and that as we push into Holy Scripture we are pushed up against an ‘ultimate’, up against God. It is from this ‘push’ that Torrance believes the homoousion becomes the key for knowing God as we encounter Him in Christ in Holy Scripture. Torrance writes:
We must now return to the fact that the homoousion was gained through hard exegetical activity. It is not itself a biblical term, but it is by no means a speculative construction, an interpretation put upon the facts by the fathers of Nicaea; rather it is a truth that was forced upon the understanding of the Church as it allowed the biblical witness to imprint its own patterns upon its mind. We can see the same thing happening in the formulation of the doctrine of the Spirit. The biblical writers nowhere provide us with clear-cut propositions as to the Deity of the Spirit but acknowledgement of the Deity of the Spirit and his inseparable connection with the Father and the Son in the Holy Trinity is forced upon the Church as it penetrates into the interior logic of the biblical witness and through it allows the inherent order and pattern of the divine Reality to impose themselves upon its mind. Theological activity, then, is not concerned merely with biblical exegesis or with a biblical theology that builds up what this or that author in the New Testament taught about the Faith; it is concerned with the Truth at a deeper level, in the necessary and coherent thinking of the Apostles as they mediated the divine revelation in Jesus Christ to the world of historical understanding and communication. Thus in formulating the homoousion the Fathers of Nicaea were penetrating into the interior logic of the apostolic witness, and allowing the truth that was embedded there to come to view in an orderly and articulate way. They allowed the fundamental nature of the subject-matter to shine through to them and to take, in their thought and speech of it, a form through which its truth could be accurately and clearly and unambiguously acknowledged. The homoousion is thus an articulation of what the Fathers of Nicaea had to think and say when they set themselves to a disciplined and objective inquiry into the biblical witness to Christ, for its basic formulation had already been given by the Apostles themselves. Hence true theological thinking is basically and inescapably apostolic, for it is determined by the form in which the Apostles handed on the Word which they themselves had received.[1]
All orthodox Christians of every stripe affirm the homoousion, but not all apply it in the principled way that TF Torrance does. In the hands of Torrance the homoousion becomes a hermeneutical touchstone by which all biblical and theological reflection must pass.
Critics of TFT will claim that Torrance has misappropriated the homoousion, and uses it in a way that the ‘Fathers’ never really intended or envisioned. But this is the genius of Torrance, I think, he constructively retrieves a grammar and construct that is present in the church’s ecumenical past and redresses it in such a way that allows the homoousion to cease being a static, albeit orthodox, pillar of the past and present, in regard to christological truth, and organically takes it and demonstrates the life that this concept has in fundamental ways in establishing a theological hermeneutic that has the capacity to bear fruit for the church in ways that magnify Jesus (the work of the Holy Spirit Jn 14–16); and at the same time offer an alternative method for theological exegesis and Dogmatic and Systematic theology that places Jesus in the center of everything in intensive and principial ways.
[1] T.F. Torrance, Theology in Reconstruction (Grand Rapids, Michigan: William B. Eerdmans Publishing Company, 1965), 40.
Athanasian Reformed 