[quote author=Archonsod]
No it isn't. An atheist has no belief since, in order to be atheist, you must reject all supernatural explanations for the universe. If you believe in something, you're not an atheist.
[/quote]
You still have problems there with strong assertions - stating that "god does not exist" goes outside of what can be empirically proved, and into the realm of belief. Of course, you can rephrase it as "I have seen no evidence of the existence of any god", but then you're in Huxleyian agnostic territory.
But then the definition of atheist will forever be the subject of unending internet arguments.
On topic - given that the devs are based in Turkey, and the increasingly extreme religious views and views against religion globally in general, as well as Turkey in particular, I totally understand why they'd take the simplest approach and just omit it. That way, no one can be offended, theists, atheists, pantheists or flyingSpaghettiMonsterists alike.
It'd be good fun to have in any attempt at a realistic Middle Ages mod though, as it would have the capacity to change the dynamic of the game considerably.