How can I avoid coming out as an atheist to my boyfriend's parents?

I'm gay and my boyfriend recently came out to his parents. They are older and pretty religious, but they are doing their best to be accepting to our relationship. However, my boyfriend says that they believe that I am changing him for the worse in that he has not been as communicative and open with them because he didn't come out to them sooner and has not been sharing the progression of our relationship with them. (The whole concept of being in the closet seems completely alien to them.) But they do know our relationship is serious, so they have invited us to spend the holidays with them in order to get to know me better. My boyfriend says that they will insist that we attend church with them and has asked that I not tell them that I'm atheist right away. I've explained to him that I am not going to lie about anything, but I am not sure how to remain true to my convictions without making things more difficult for my boyfriend and upsetting his parents. What are your suggestions for making the Christmas holidays pleasant while maintaining my integrity?