Christ, in many Christian belief systems, is God. According to both Matthew and Mark his last words on the cross were: 'My God, My God, why have you forsaken me? (Matthew 27:46 and Mark 15:34). The question is this: If Christ is God, why is he questioning his own methods and motives? Surely to question God's methods and motives is to question His omnipotence and benevolence because you are doubting whether He is sound in His actions, 'perhaps He (I) made a mistake, Surely He (I) didn't intend to let me die in this manner; He has abandoned me...' If we accept that God is an omnigod then he cannot make mistakes, to question him is to question whether he is an omnigod and if God must be an omnigod then He cannot exist at all (given that He makes mistakes). He who questions the methods and motives of an omnigod must, disbelieve that the omnigod exists at all, and for certain Christians he can only be an omnigod.
Now if God is infinite then what happens in a split second in God's mind is eternal. What is true of God in this minute is true of Him forever; if God changes then He was not perfect before, perfection needs no change. Therefore in this brief moment, the last of Christ on Earth, God was an atheist, and must therefore be an atheist forever.
There is, as always, a response to this. We could say that in becoming man God lost all his power and knowledge of events to come and the workings of the universe. Christ is not akin to his own methods and motives and is questioning the Him that Knows. Yet Christ, according to the Gospels, is well aware that he must die and indeed why he must die. Who knows... perhaps, experiencing as much pain as his flesh and blood incarnation could take, he, as a finite man, a flawed man, he forgot his own omnipotence and benevolence... perhaps he forgot that he was God...
I've tried to keep this brief and in doing so may have lost the force of Zizek's point. It has also been a while since I read the book and I'm unsure what Zizek's ultimate point was. I think it was that God longs to be human; that in being finite, emotional and flawed he becomes whole. Or perhaps it was about the paradox of the situation; Zizek's ubiquitous: 'I know the truth but...'
Anyway, if you get the chance give it a read. It's great