"...at the decisive points they cannot fail to hear something of the rolling thunder of the 1921 Romans..."
So says Barth in the preface to CD IV/2. The "they" he is referring to are the Pietists concerned with the doctrine of sanctification (and the apparent lack of one in Barth's theology). IV/2 is Barth's answer to this group, though it is an answer which may not satisfy them. Why not? Because the "thunder" of Romans II which so disturbed many of its readers can also be heard here in the "mature" Barth.
I mention this only because there's a great quote from Deadwood (delivered without suitable gravitas by the inimitable Keith Carradine) which captures what I'm trying to do with my thesis on Barth's concept of love: "listen to the thunder."