Just for the sake of confusing philisophical discussion, I'm going to introduce:

MrDevious' Wager:

There are alien beings in this universe of astounding power. They can travel to any point in this universe in a matter of seconds. They can destroy worlds with the utmost ease, they can whipe out galaxies if it so tickles their fancy.

And it just so happens, they don't believe in god. In fact, they are so offended by the notion of god that they destroy any and all alien life they encounter that worship a diety. While there are billions of galaxies full of life-inhabiting planets, they'll eventually come across ours.
So, whether or not it may be rational to believe in these creatures, can we really take that chance? It's therefor only reasonable that we take the safe road and stop believing in god, lest the creatures should find Earth one day and kill us all.