I know this may seem like semantics, but it does create some of the ignorance in this debate.
It would make no sense to be an atheist in the Clash of the Titans world. There are gods all around directly involved in human affairs. To not believe in them would be silly, they're real, they can summon Krakens. To defy the gods and kill them is anti-theism, and there is a big difference.
Atheist is to theist as someone who does not collect stamps is to stamp collector. All it means is you don't collect stamps, it states nothing as to your feeling about stamp collecting. To be "militant" about not stamp collecting would be silly. The only issue would be if you were forced to collect stamps, and desiring to not to be compelled to do something is just exercising a basic right.
On the film, looks like a rental someday.