Okay, well, in light of your last post, it seems that she will not rest, or let you rest, until she gets a lot off her chest.
So, let her. Be like the woman on a bicycle who was pedaling so fast to try to outrun a thunderstorm, and finally realized that if she just stopped trying to run away from it and let it pass over her, it would soon be gone.
She really, really needs to feel that you've heard her out on this. Do you have to? No. Is it the shortest route to getting past that? Very likely yes. And this is an important subject, and you're her daughter -- not just some stranger she is trying to meddle with. It's really not so unreasonable for her to want to express herself to a pretty thorough degree. Remember, there may come a day when she gets involved with, I don't know, some sort of business thing that you fear may be a pyramid scheme or something, and you're going to want her to listen to you.
So maybe you can say something like, "Mom, I can see this is really, really bothering you, and you have a lot you want to say to me. And I want to listen. But I get frustrated when we go over and over the same ground, and anyway, this subject deserves more than quick over-the-breakfast-table or during-the-commercial talks. So let's get out our calendars and find a time when we can have a quiet lunch [or whatever you want], just the two of us, where I can really listen to what you have to say, and you'll have time and no interruptions. And then we won't keep going around in circles and driving each other crazy. Okay? How does Tuesday morning look?"
Get it? Forget about trying to explain your religion to her just now or asserting your autonomy. Just LISTEN. You don't have to agree or obey or promise or change. But listen. Forget about trying to avoid hearing what she wants to say; in fact, you'll need to be sincere in your offer to listen, and then do it. I don't mean listen in the sense that if she says Religion X kicks puppies, you have to agree with her (although I'd avoid the pointless argument, just get past it fast). I mean listen to her FEELINGS. Let her know you truly heard her.
Not only is that a good idea for any relationship, if you do it, THEN you will be in a much stronger position to say, when (inevitably) she starts at you again, "Mom, I know you're only speaking out of caring, but this really is wearing on me, and we aren't going to get anywhere new. I really did hear what you had to say when we had that four hour lunch last May, and I really do appreciate your concern. But now it's enough; any more isn't going to change anything. Please, let's not waste our time together going over this same ground again."
But all this presupposes that you actually want to have peace here. Are you more interested in having a battle and winning it? Many people are, and it doesn't make them bad people. You seem not to like the advice you get from posters who tell you other than "cut her off" or "get in her face." We don't know her; you do. So it's not like we can say "preserve the relationship at all costs" -- how do we know whether you should do that? But people here are trying very hard to help you, and I think that the reason you're not getting what you want is that your objectives still aren't clear, at least to us. You need to think about and clarify -- not for us, but for yourself: exactly what is it that you want to achieve?