How to tell Christian boyfriend that I'm atheist

My boyfriend and I have been dating for over 2 months now and we have religious and political difference but we avoid talking about them. I think it's important for him to know however that I am atheist because he is pretty religious. He only knows about our political differences and I don't know how he will react to the fact that I don't believe in God. I respect his religion I just don't want my atheism to offend him, and if it does I will not respect him. I respect people who have respect for others' values. Sooo, any suggestions on how to break it to him??