Barth, not Popper.
There is no doctrine more dangerous than the Christian doctrine of the atonement, it does indeed make “wild and careless folk” (Heid. Cat., qu. 64), if we do not consider it with this warning in view. The fact that it speaks of God making good what we have spoiled does not mean that we can call evil good (unless we would also call good evil). All our thinking and all that we say on this matter must be disciplined by an observance of this limit, and a refusal to transgress it in any circumstances, sense or direction.
Simply, those who proclaim “felix culpa” make a category error. The fall was not, and cannot be happy. To suggest that God’s making evil good means evil is good is to misunderstand the very nature of atonement, or so says Karl Barth.
I’ve recently had my baptism into Barth and have not been disappointed, especially since I get to read him with one of the finest minds I’ve ever encountered. The small slice of Barth’s theology (on the Reconciliation) I’ve read has been complex and dense, but compelling. Barth’s work needs a warning label–it is for the theological sophisticate, and shouldn’t be tackled without the help of someone acquainted with his work. However, if anyone wants heart-warming, intellectually robust theology that fits in the tradition of Calvin, Barth is the guy to go to.