The homoousion is the key piece of language the council of Chalcedon borrowed from the Greeks in order to put it in the service of grammarizing a way towards speaking of the reality of the eternal Son, Jesus Christ, as being exactly consubtantial with the Father (and Holy Spirit) in nature as Theos (God). It was also used dually to speak of the reality of the eternal Son as being consubstantial in nature with humanity; which resulted in the further language of hypostatic union. That’s my very rough entrée into introducing you all to the homoousion. The reason, in particular why I’m bringing this up is because I want to highlight something very important in regard to what serves as a touchstone for T.F. Torrance’s theological realism and hermeneutic; he places central weight upon the homoousion as the basis upon which humanity has touch with God and God with humanity—so not only is there an epistemological component to it, but antecedently and as a prius there is ontological import, for TFT, in emphasizing the significance for a truly Christ conditioned/centered reality. Whether that has to do with hermeneutics, a doctrine of creation/re-creation, the eschaton, or what have you.
I want you, the reader, to see how this works in Torrance’s theology, and in so doing I want you to see what stands behind my own approach to all things theological; it is in echo of Torrance. I want you, the reader to understand how important the homoousion is for my and Myk Habet’s understanding of what Evangelical Calvinism entails, and is, indeed, entailed by. Here is what Torrance writes about the homoousion, in brief:
As the epitomised expression of this truth, the homoousion is the ontological and epistemological linchpin of Christian theology. It gives expression to the truth with which everything hangs together, and without which everything ultimately falls apart. The decisive point for Christian theology, and not least for the doctrine of the Holy Trinity, lies here, where we move from one level to another: from the basic evangelical and doxological level to the theological level, and from that level to the high theological level of the ontological relations in God. In that movement a radical shift in the basic fabric of theological thought takes place along with a reconstruction in the foundations of our prior knowledge. This is evident not least in the fact that in formulating the homoousion of Christ in connection with both his creative and redemptive activity, Nicene theology laid the axe to the epistemological dualism latent in Greek philosophy and religion that threatened the very heart of the Gospel; and as such it gave powerful expression to the indissoluble connection in Act and Being between the economic Trinity and the ontological Trinity, between οἰκονομία and θεολογία, which secured the Church in its belief that in the Lord Jesus Christ and his Gospel they had to do directly with the ultimate Presence and downright Reality of God himself. Jesus Christ does for us and to us, and what the Holy Spirit does in us, is what God himself does for us, to us and in us.
There are many things that could be noted in regard to the various high points evinced in this one paragraph from Torrance. But let me just highlight a couple: 1) We see how Torrance believes (and I’m with’m) the Hellenic language of homoousion was taken and ‘reminted’ (that’s his language from just prior to this paragraph) under the pressure and reality of the Revelation of God in Christ. He believes that the way the early church was able to appropriate this language was in a way that re-texted said language to the point that it became a brand new, even “Christianized” grammar that was made fitting for the church’s edification precisely because of the power of God in Christ as the coordinating reality of all things; even language. 2) The homoousion for Torrance, as we already noted, has both ontological and epistemological import for us because it shows how the Godward movement towards us is in intimate, even perichoretic relation to the humanity he assumed, and how the Humanward is in intimate relation with Godself which is the ground of the singular person, Jesus Christ. It is as this hypostatic union inhered in Jesus by the creative power of the Holy Spirit that the bridge between God and humanity/humanity and God was accomplished. It is this reality, the personalizing personal reality of God in Christ, that as we participate in and from it, from Him, that we now have access to the holy of holies of God’s life; we have both the ontic (being) and epistemic (knowing) capacities necessary to actually have access to God’s life (Eph. 1.18-9)—the Holy Trinity, mysterium Trinitatis!
 Torrance has what he calls a stratified knowledge of God, that’s what he’s referring to here in regard to the ‘levels’. I’ve hyperlinked to a brief quote from Ben Myers who gives a nice summary of what this is all about in Torrance’s theology.
 Thomas F. Torrance, The Christian Doctrine of God: One Being Three Persons (London: Bloomsbury T&T Clark, 2016), 95.