It all depends. An ultimatum wouldn't really be effective on her part. You believe what you believe. I myself don't follow any religion and I know it makes some people uncomfortable. I think she has a vision of the perfect husband/family in which religion is of importance. She sees your lack of faith as an incompatibility. How religious are her parents? Maybe she also feels pressured because they would not approve of your beliefs. YOU need to make her understand that there is nothing that can be done about it and to either accept it or not. I'm willing to bet that she would make your kids Christian anyways, so everything will play out the same as if you belonged to a religion besides Christianity. Maybe she just thinks your going to hell or something.
