Misfitmel
One thing to keep in mind is that you are dealing with two distinct, separate issues that ought not to be confused.
The first is your religous beliefs or the lack thereof. Frankly, that is a very personal matter that we may or may not chose to share with others, even our parents, but it is very much our choice.
The second is boundaries in the relationship with your mother.
I'd say you need to get the second right before you even begin to consider the first.
Don't start off talking about your ideas on religion, start off discussing whether you have a right to your own opinions. Establish a level of respect for each others ideas before any discussions begin.
I had a boss who would randomly decide to micromanage an operation. He was authoritative and aggressive, but often right. I was responsible for designing a new laboratory and in a meeting we me and my direct boss, he wanted to pass judgements on my decisions. My first question to him was "Who decides? You or me?" Boy, did the sparks fly. But we established that he did not want to be involved in every little minor decision and that he would have to trust me. Once done, he was pleased with the result. Lesson being that we could not have an intelligent discussion until we had the roles established.
Establish the boundaries with your mom, then get to the religion. But if she won't accept the boundaries, any discussion is doomed to frustration and probably anger on both sides. That would be unfortunate. Sounds like you two need each other.
Best Regards
Damocles