Polite Pretense
I'm starting a new religio-scientific movement. We argue that although God created the universe in a week a couple thousand years ago, He included such an enormous and painstaking amount of evidence for billions of years of history that it would be downright rude to not act fooled. Just as courtesy demands that you act surprised at a surprise party, even if you saw it coming, only a boor would acknowledege that he sees through the evidence of an old universe.
UPDATE: Lore has his own competing theory