Why the apocalypse must be human made?

In the Preacher series, God and The Grail organisation are working on bringing the apocalypse. But even while God keeps saying that "he could do it at any time by himself" (he has done it with dinosaurs already), he insists that it must be a man-made one, started by accepting Humperdoo as the new Messiah (by admiring his tap-dancing skills).

So why exactly is waiting for The Grail to start the nuclear war? Was that supposed to be an Old Testament-style test?

^{I understand that both the series and this question might be bit controversial, please note that I am not trying to offend anyone here ;)}