Thomas Torrance is known for his deployment of the homoousion, the grammar developed primarily at the Council of Nicaea in 325ad. It is the attempt to articulate how it is that Jesus, the eternal Logos, and Son of God is eternally consubstantial and of the same ‘substance’ or better ‘being’ (ousia) with the Father [and the Holy Spirit]. It is this idea that Athanasius, particularly after the Council of Nicaea went on to develop and argue for in his engagement with Arius et al. This serves as a key piece for all orthodox Christians because it helps us double down on what is revealed in Jesus Christ about himself as the Savior of the world, and how that is, as he is eternally Son of the Father. This doctrine is significant because it identifies the Trinitarian structure of the Gospel, and demonstrates how it is that the Son must be God, not just man (i.e. against Ebionite Christology and any other adoptionistic thinking), if in fact he would actually have the capacity to ‘save’, to redeem, to reconcile humanity unto God. This doctrine also is significant because it goes both ways, it not only positively notes the Son’s eternal relation to the Father as his one and only begot, but it also does double duty by pressing home the fact that he, the Son, is also fully consubstantial with human being; i.e. that he is fully human. Here the ‘bridge’ is realized between God and humanity, as the Son assumes flesh for himself, and in so doing becomes the Mediator between God and man (cf. I Tim. 2:5-6). It is in this reality, the homoousial reality that the gap between God and humanity, because of not only our finitude, but also our falleness is remedied; and we are brought from our lowly fractured state and elevated to God’s kind of life, not by nature, but by the grace of God who is Jesus Christ. It is because of the homoousial reality that we, as the Petrine theology asserts, are brought into the divine nature as participants through the grace of God’s life in Jesus Christ for us and with us. And it is because of the homoousial reality that any type of dualism between God and humanity is mitigated and brought into unity of both being (ontology) and thought (epistemology) as Jesus mediates God’s life to us, and our lives to God’s triune life in and through his life with the Father by the Holy Spirit. Because of all of this, and more, Thomas Torrance writes this about the importance of the homoousion:
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.
As Torrance highlights when we see the Father we see the Son; i.e. the ontological inner life of God (in se) is really made known in the economic outer life of God (ad extra). If it wasn’t, as Athanasius would argue, we are of ‘all men most to be pitied;’ because if true God of true God did not come for us then we would be doomed and left to ourselves in our sins. The gap between the Creator, who has always already been Father, Son, and Holy Spirit, and the creation (humanity) was so great that if God did not stoop down to us in the grace of his life in Christ we would forever be in our sins and creation itself would be lost. This is what Arius’s theology entails; i.e. since his view of the eternal Son [Jesus] is that he is generate, meaning a creature; if this was so all that humanity would be left with in this scheme, soteriologically, is a salvation that remains contingent upon us to ‘work out our salvation’ in such a way that we might find merit before God. Jesus becomes an instrument or exemplar in the Arian way of Christology and soteriology, such that there is no bridge, no Divine mediation between God and humanity; there is no union of God and humanity and humanity and God in Arian theology. Athanasius would go on and show how the homoousion undercuts this faulty way of Arian thinking both theologically and biblically. Thomas Weinandy explicates how this worked out, in Athanasius’s theology, and how the homoousion functioned as key for providing an orthodox understanding of salvation (Thomas G. Weinandy, Athanasius: A Theological Introduction (Hampshire, UK: Ashgate, 2007), 63-4.):
As Weinandy has demonstrated without the homoousion, in Athanasian and orthodox theology, Christianity may have failed. We might still be in our sins. We must believe the Dominical teaching here when Jesus proclaimed that the gates of hell would not prevail against the Gospel reality (cf. Mt. 18); we must acknowledge God’s providential care in providing people like Athanasius for his church in seminal and early ways. Without such guidance we could only imagine where the church might be today.
Thomas Torrance understands all of this, and this is why he has made the homoousion key to the whole of his theological program. As he once said of himself: “I’m an Athanasian, if anything” (my paraphrase).
 Thomas F. Torrance, The Christian Doctrine of God: One Being Three Persons (London: Bloomsbury T&T Clark, 2016), 95.