It's true that something like "1+1=2" is not a true statement a priori, it's a consequence of logical extensions of mathematical axioms ("assumptions"). However, in the case of arithmetic (though not necessarily other branches), these axioms are designed to approximate our experience with the physical world, and consequently the system which arises from them is capable of making successful predictions about the world. This is what separates it from religion in this respect.

The religious would argue that their religion makes successful predictions about spirituality. In the idea that if you pray for answers you may receive answers, and if you don't receive answers you aren't praying properly, just like if you don't receive the correct answer with mathematics it is a calculation error. While it's easy to show the practical applications of mathematics, because as stated it is meant to deal with experience in the physical world, the religious deals with "the spiritual world" and as such their predictions deal with "spiritual aspects". A bunch of bullocks, I know, but that's the basis of the comparison.