My boyfriend's mom doesn't like me because I'm not Catholic?

My boyfriend and his family are hardcore Catholic. I'm basically agnostic. I don't go to church or anything. His mom isn't mean to me or anything, and she's polite to my face, but just from the way she acts around me and all the subtleties, I can tell she doesn't approve. And it hurts. I hate being the outcast in his family. She loves her other son's girlfriend, who is Catholic. What can I do for her to approve of me being with her son? Or will she just be cold with me forever?
My boyfriend's mom doesn't like me because I'm not Catholic?
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