Seems like Zizek has read a little Barth (specifically from God Here and Now, which was one of the first books I read by Barth). From his most recent books, Zizek writes:
"With Christianity, it is the obverse-not "God proposes, man disposes," but rather, "God (first) disposes, (and then) man proposes." What this means is that, although the Event has already happened, its meaning is not decided in advance but is radically open. Karl Barth drew the consequences of this fact when he emphasized how the final revelation of God will be totally incommensurable with our expectations:
God is not hidden to us; He is revealed. But what and how we shall be in Christ, and what and how the World will be in Christ at the end of God's road, at the breaking in of redemption and completion, that is not revealed to us; that is hidden. Let us be honest: we do not know what we are saying when we speak of Jesus Christ's coming again in judgment, and of the resurrection of the dead, of eternal life and eternal death...continues long quote...see pages 45-6 of God Here and Now).
See God in Pain pages 223-5 for Barth references
