Girlfriend said she would leave me if I didn't convert to Christianity?

My girlfriend of a year recently approached me and told me that unless I believed in her christian god she would have to leave me. This was very upsetting as I love her dearly. I have been accompanying her to church every Sunday to show her that I support her but it doesn't seem like it is enough for her. I have a very strong mind and I am not the least bit credulous. It seems that she wants to change me with the threat of leaving me - I did let her know that I would try to take it more seriously but couldn't find a way to tell her that I wasn't going to give up my principles for something I am not capable of believing in, I needed more time to think. Over the past few weeks since this illumination things have been going down hill; I am not my cheery self around her nor does she make me feel like my efforts to show my love to her are worthy of her attention, I feel - in a way - heart-blocked by god. What should I do? I don't want to lose her, we are great together but now I feel like she is constantly silently judging me. It all feels like a black cloud looming above me....