Sam can believe in angels and a 'good' higher power for all he wants, but Dean knows better. If there is a God – and he's not saying there is – wouldn't he had stopped The Demon from burning his mother to death? And if he didn't, doesn't it make him just as bad as the so-called devil? (Dean isn't sure about that guy, either, but his existence seems to him more likely than that of the other one).
Layla's 'Maybe God works in mysterious ways' comes to his mind each time when he thinks that, and each time he decides that 'mysterious ways' equals 'let's screw the Winchesters a bit more.' God's mysterious ways led, so far, to the deaths of his mother, his brother's girlfriend, father, and him selling his soul to a demon to save his brother from the same fate.
So yeah, Sam can believe all he wants. Might as well start to believe at the tooth fairy again while he's at it. Dean believes it's up to them to save the world. After that, it will be just him and the hellhounds.
Mysterious ways, his ass
A/N
Yeah, blasphemy. Don't say I didn't warn you.