As an evangelical in Bible College and Seminary (I still consider myself, broadly construed, an ‘evangelical’) I held to the impeccability view of Christ’s humanity. In other words, I believed that not only could Christ not sin*, but that the body he assumed in the man, Jesus of Nazareth, was likewise uniquely fitted for him such that he did not enter into the fallen human nature that the rest of humanity is born into in their mother’s womb. But then later, after Seminary (I graduated in 2003), as so many of you know by now, I came across the writings of Thomas Torrance; Torrance, as many of you also know holds to the Athanasian idea that Christ, in the incarnation, assumed a fallen human nature, just like the rest of humanity’s. Along with Nazianzen and Athanasius et al. Torrance maintained that unless Christ fully entered into our real and fallen human nature that real redemption, all the way down, could not take place. Torrance would be concerned, also, that if Christ didn’t enter the fallen human nature, in the assumptio carnis, that all we would be left with would be with something like an instrumentalist conception of the atonement. I.e. We would be left with a forensic understanding of salvation, necessarily so, since the death of Christ wouldn’t penetrate deep enough into the fabric (ontologically) of human nature to recreate it, but instead he would only be the ‘organ’ of God’s salvation to ‘pay the penalty’ of humanity’s sin (in particular the elect’s); a truly juridical and external type of venture.
Here is what Torrance has written in his New College lecture notes:
Now when we listen to the witness of holy scripture here we know we are faced with something we can never fully understand, but it is something that we must seek to understand as far as we can. One thing should be abundantly clear, that if Jesus Christ did not assume our fallen flesh, our fallen humanity, then our fallen humanity is untouched by his work — for ‘the unassumed is the unredeemed’, as Gregory Nazianzen put it. Patristic theology, especially as we see it expounded in the great Athanasius, makes a great deal of the fact that he who knew no sin became sin for us, exchanging his riches for our poverty, his perfection for our imperfection, his incorruption for our corruption, his eternal life for our mortality. Thus Christ took from Mary a corruptible and mortal body in order that he might take our sin, judge and condemn it in the flesh, and so assume our human nature as we have it in the fallen world that he might heal, sanctify and redeem it. In that teaching the Greek fathers were closely following the New Testament. If the Word of God did not really come into our fallen existence, if the Son of God did not actually come where we are, and join himself to us and range himself with us where we are in sin and under judgement, how could it be said that Christ really took our place, took our cause upon himself in order to redeem us?
And here is what theologian Thomas Weinandy has to say about Athanasius’ view on the same loci (Thomas G. Weinandy, Athanasius: A Theological Introduction (Hampshire, UK: Ashgate, 2007), 33-4):
It is easy to see the connection between the incarnation and salvation, in ontological terms, when we consider it from both Athanasius’ and Torrance’s theo-logic. It happens to be a theo-logic I affirm these days. Indeed, there are many objectors to this (Kevin Chiarot being the foremost) thinking; but it would be wrong-headed to think that there is not some seminal footing for this view from none other than the champion himself of Nicene Christology and Trinitarian theology, the theologian contra mundum, Athanasius.
It should also be kept in mind that this is precisely the point at which departure happens between Evangelical Calvinists and Classical Calvinists. Classical Calvinists frame their understanding of salvation, primarily, within forensic/juridical lenses; this flows well and even from their understanding of the Covenant of Works combined with the God of absolutum decretum (the God who relates to creation through absolute decrees), and a doctrine of unconditional election. Evangelical Calvinists follow Athanasius, Torrance, et al. in adopting this more ontological understanding of salvation wherein the primacy of Christ as the imago Dei is elevated to the point wherein salvation is understood as the realm where humanity is taken up in the assumption of God’s humanity in Christ, and we are recreated in Christ’s resurrected vicarious humanity for us (Romans 6–8); we are taken from living in subhumanity and corruptibility and brought to participate in and from the incorruptibility of God’s life in Christ, the life that is indestructible (Hebrews 6–7). We still see the forensic in the atonement, but we emphasize, with Barth, the idea that God in his election to be for us in Christ becomes the judged Judge in our stead and reconciles and elevates humanity to be partakers of the divine nature (by the grace of his life) in and through the recreated humanity of Jesus Christ who is our mediator.
 Thomas F. Torrance, Incarnation (DownersGrove, IL: IVP Academic, 2008), 62.
*To be clear, I along with Athanasius and TF Torrance do not believe that Christ ever sinned, but immediately sanctified his humanity, by the power of the Holy Spirit, remaining the spotless Lamb of God who has taken away the sins of the world. Such a sacrifice was required in order for actual salvation to inhere; which is of course why it took God in flesh, the double homoousion of the Son as God, and the Son as human in the singular person of Jesus Christ to accomplish such an impossible possibility.