Atheists admit the truth of many things that are unprovable. For example, Euclid's postulates in geometry.
Atheists need books to convince themselves why the postulate of an Unmoved Mover or First Cause is untrue, while Euclid's postulates in geometry are true*. Neither are provable, however. http://dictionary.reference.com/browse/postulate
[v. pos-chuh-leyt; n. pos-chuh-lit, -leyt] Show IPA verb, -lat·ed, -lat·ing, noun
5. something taken as self-evident or assumed without proof as a basis for reasoning.
6. Mathematics, Logic . a proposition that requires no proof, being self-evident, or that is for a specific purpose assumed true, and that is used in the proof of other propositions; axiom.
7. a fundamental principle.
8. a necessary condition; prerequisite.
*Actually, they aren't all true, but that's for another thread.