Hm, that's a tough one. Obviously you value her friendship, and I'm sure she values yours as well. I imagine at the heart of it, she believes that "God", "faith", whaterver it is that she has found to help her, is just the ticket to help you. I must confess, I've gotten caught up in the touchy-feely email circuit myself, but it's guilt rather than love that motivates the continuation of them for me. HOWEVER, I would NEVER forward one to someone who I didn't already know,
A. Had some beliefs along that line and thought that they would get something out of it, even if I don't, and
B. Was one of the people who forwards this kind of stuff themselves.
Anyway, personal choices aside, maybe sending back these emails with an "auto-responder" type message, stating that this sort of forward is not welcome or accepted at this address. Something impersonal, unharsh, friendly-business like. It's not just a message to your friend, but to anyone, present and future. I don't know if you feel comfortable with that or not, but it's a way to deal wiht it globally, and not make it a personal thing about your friend - you can thank her for prompting you to finally get around to doing something like this.
Claire's mention of using rules is just the thing to use to facilitate this. You can set up a rule to scan for the words "Christ", "prayer", and "saviour". As they continue to slip through in lesser number, you can add words to scan for to make your filter more efficient.