Ultimately, though, the warlock isn't investing any faith or belief in his patron. He is essentially entering into a business deal. He may be in complete disagreement with his patron's motives, and yet still be willing to trade with it.
I'm also a fan of the "approached in a moment of weakness and tricked into making a bargain" backstory. I've got a few warlocks who actively work against their patron (or think they do - is it all part of the plan?)