Marion Cotillard was born in Paris, France and grew up in Orleans, France.
There is almost no information whatsoever on Cotillard's religious views. One commenter on a question & answer site of questionable credibility seemed confident enough to say that Cotillard is a practicing Catholic.
But that's certainly not enough to make a call, even if it would make sense. France is a traditionally Catholic country with 81% of French citizens declaring themselves Catholic as recently as 2010.
One could make the assertion that Cotillard simply isn't religious, but one thing to realize about many Western Europeans: they just aren't as vocal about religion as the rest of the world.
Either way, she's not oblivious to religion. She's mentioned that she will take into account and study the religion (among other things) of her characters when preparing for a role.
Follow the money
I have a tendency to often share the point of view of the conspiracy theory.
Of the September 11th World Trade Center attacks, Cotillard said:
I think we're lied to about a number of things. We see other towers of the same kind being hit by planes. Are they burned? There was a tower, I believe it was in Spain, which burnt for 24 hours. It never collapsed. None of these towers collapsed.
She went on to explain that the owners of the towers probably didn't want to foot the bill to modernize the old buildings, so they opted to have them destroyed in a dramatic display of fire, destruction and death. But that's not all. Of the U.S. moon landing, Cotillard said:
Did a man really walk on the moon? I saw plenty of documentaries on it, and I really wondered. And in any case I don't believe all they tell me, that's for sure.
We've certainly got ourselves a radical here.
In mainstream sociopolitical causes, Cotillard prefers environmentalism. She was actually educated an ecologist and she expresses it as a highly-regarded member of Greenpeace, traveling the world on missions for the organization.
Agree or disagree, Cotillard is certainly an interesting, thoughtful person.