I think something that is not talked about much, in regard to Barth’s theology, is how counter it is to mediaeval conceptions of salvation and grace relative to a grace/nature binary. In other words in the major strand of Western conception of salvation/grace we get
something as definitive as Thomas Aquinas’s axiom: ‘nature is perfected by grace.’ So we have this kind of symmetry between the two with a telos (or linear purposiveness) tied into nature by God’s grace coming along, as it were, and completing or bringing nature to where it inherently has been designed to be. In this scheme we have what some have called a ‘pure nature’ (naturum purum); what is implicit in this scheme is that at the fall nature only sublimated into a sub situation relative to its inherent trajectory before God—in other words, nature did not fully self-destruct into demise and utter death through and through; a spark of its inherent determinancy remained hither. It simply needed help-along by way of God adding to it his created grace wherein nature, and the stewards of said nature, human beings, could habituate in this added grace in their lives thus bringing nature to where it had always intended to be at an inherent level (as originally designed by God).
But for Barth this is not how nature/creation is conceived of to begin with. There is no nature/grace symmetry; for Barth it is ‘grace all the way down’ (to use a Torrancism). In other words, the condition for creation itself is grounded in God’s first Word of grace realized in his elected life in Christ to be for us and with us. Grace is the precondition of creation for Barth such that nature has no inherent determinancy or ‘purity’ in itself. We might note something like this, from Barth, as a counter to the Aquinisian axiom we shared above: ‘creation is the external basis of the covenant’ and ‘covenant is the internal basis of creation.’ What this gets at in our discussion is how for Barth, contra Aquinas&co., creation/nature itself is inherently tied into God’s gracious choice to be for us in Christ; Aquinas’s view has nature tied to grace in a kind of complementing sense whereas Barth sees creation/nature as always already a reality that is thoroughly suffused and conditioned by and from God’s life through and through.
I bring all of this up to lead us to a quote from George Hunsinger on Barth’s theology in regard to salvation, human cooperation in that salvation (and not), and human agency/freedom. Maybe you will see how my rough sketches on Aquinas juxtaposed with Barth fits into what Hunsinger is getting at in regard to how distinct Barth is from the trad on this most crucial point. At length, Hunsinger writes this per Barth:
Human Cooperation Does Not Effect Salvation
Barth does not deny that human freedom “cooperates” with divine grace. He denies that this cooperation in any way effects salvation. Although grace makes human freedom possible as a mode of acting (modus agendi), that freedom is always a gift. It is always imparted to faith in the mode of receiving salvation (modus recipiendi), partaking of it (modus participandi), and bearing witness to it (modus testificandi), never in the mode of effecting it (modus efficiendi). As imparted by the Spirit’s miraculous operation, human freedom is always the consequence of salvation, never its cause, and therefore in its correspondence to grace always eucharistic (modus gratandi et laudandi). These distinctions apply both objectively and subjectively, that is, not only to salvation as it has taken place extra nos, but also as it occurs in nobis. Since to be a sinner means to be incapacitated, grace means capacitating the incapacitated despite their incapacitation. Sinners capacitated by grace remain helpless in themselves. Grace does not perfect and exceed human nature in its sorry plight so much as it contradicts and overrules it.
What happens is this: in nobis, in our heart, in the very center of our existence, a contradiction is lodged against our unfaithfulness. It is a contradiction that we cannot dodge, but have to validate. In confronting it we cannot cling to our unfaithfulness, for through it our unfaithfulness is not only forbidden but canceled and rendered impossible. Because Jesus Christ intervenes pro nobis and thus in nobis, unfaithfulness to God has been rendered basically an impossible possibility. It is a possibility disallowed and thus no longer to be realized . . . , one we recognize as eliminated and taken away by the omnipotent contradiction God lodges within us. [Karl Barth, “Extra Nos-Pro Nobis-In Nobis,” Thomist 50 (1986): 497-511, on p. 510.]
In this miraculous and mysterious way, by grace alone — that is, through a continual contradiction of nature by grace resulting in a provisional “conjunction of opposites” (coniunctio oppositorum) — the blind see, the lame walk, and the dead are raised to life (cf. Matt. 11:4).
Do you see the type of discontinuity and asymmetry that is present in Barth’s understanding of a doctrine of creation/nature, and how that implicates the way Barth conceives of what happens in the salvific reality? In the Thomist account grace attends to nature in such a way that nature is prolongated to new heights, but heights pregnant within nature itself. In the Barthian account creation had no such inherence, nature was always already extrinsically conditioned by and for God’s life of grace in Christ. For Barth nature does not simply subsist as inchoate reality waiting to be completed in accord with its own independent ends (i.e. already built-in through secondary causation etc), but instead it has always had this type of apocalyptic eschatological hue to it such that the first creation while anticipative of things to come, heightened and intensified by the fall of Adam and Eve (and thus humanity), was conditioned to be contradicted and recreated in accord with its gracious and given purpose determined by an immediate corollary between its given reality and the reality given to it, always and already, in God’s choice to be for us. In other words, in the Barthian account, to state it brusquely: creation is and always has been a predicate of God’s gracious and Triune life (insofar as he chose this to be the case). Contrariwise, in the Thomist account we could say that: grace is understood as a predicate of nature insofar as grace is seen as a supplement to expand nature to new heights; that nature came to be, as it were, fitted for grace and grace for nature. In the Barthian account nature has always been inclusiastically situated in and from God’s life of grace both protologically and eschatologically. If this is so, the Barthian account, we can see how Barth could and would draw such a brightly colored line between his own understanding of the nature of salvation versus something like we find in the Thomas Aquinas frame. For Barth first creation and second creation were always and only conditioned by God’s primal choice to be the Yes of creation from the beginning and end in Christ.
Let me close with a quote that I’ve shared before in regard to Barth’s understanding of history relative to resurrection and what that implies relative to all the realities we have just been sketching through:
A large number of analyses come up short by dwelling upon the historical question, often falsely construing Barth’s inversion of the order of the historical enterprise and the resurrection of Jesus as an aspect of his historical skepticism. For Barth the resurrection of Jesus is not a datum of the sort to be analyzed and understood, by other data, by means of historical critical science. While a real event within the nexus of space and time the resurrection is also the event of the creation of new time and space. Such an event can only be described as an act of God; that is an otherwise impossible event. The event of the resurrection of Jesus is that of the creation of the conditions of the possibility for all other events, and as such it cannot be accounted for in terms considered appropriate for all other events. This is not the expression of an historical skeptic, but of one who is convinced of the primordiality of the resurrection as the singular history-making, yet history-delimiting, act of God.
What is key in this quote is the emphasis placed on the ‘primordiality of the resurrection as the singular history-making, yet history-delimiting, act of God.’ Creation/nature for Barth is not a linear thing, it is apocalyptic; as such it is always open, contingent upon God’s own freedom, to be re-ordered and recreated in such a way that it corresponds to who he has chosen to be in Christ for us. In other words, nature for Barth, in an order of consideration does not precede God, and thus determine how it ought to be completed by God’s grace; no, for Barth, God’s choice has always been the Predicator of the predicated and Christ conditioned reality that we identify as ‘nature.’ Human freedom in salvation, in this Barthian scheme, then, can only be construed by thinking it from the conditions of this type of Christic reality in regard to creation; i.e. through its suffuse predication by what it means to be ‘free’ before God as participants in that life, and in that type of freedom, the freedom that the Son has shared with the Father by the Holy Spirit in the Ultimacy and Intimacy of their Divine life. In Barth’s scheme we shouldn’t think of nature being conflated with some sort of created grace from God, by which the elect might cooperate with God in attaining the perfection for which their created natures have always been regnant; nein, we ought to understand that God’s grace is personal and oriented always already encountering us over again afresh and anew in the face of Jesus Christ. We live from the freedom of God’s life in Christ, and this is what it means to be human; to live from the resurrected and vicarious humanity of Jesus Christ. Nature isn’t being perfected by grace in this scheme, it instead is realizing what it has always already meant to be creatures created in the image of the image of God (who is Christ cf. Col. 1.15).
I left many threads, once again, dangling. But hopefully you’re at least getting a sense a feeling of where things are going here at The Evangelical Calvinist. And maybe you will better understand why I am so resistant to classical theologies, Protestant and Roman Catholic, that work from the Thomist (neo or not) categories of ‘nature perfected by grace’.
 George Hunsinger, Disruptive Grace: Studies in the Theology of Karl Barth (Grand Rapids, MI/Cambridge, UK: William B. Eerdmans Publishing Company, 2000), 165-66.
 Robert Dale Dawson, The Resurrection in Karl Barth (UK/USA: Ashgate Publishing Company, 2007), 13.