Just say politely that you are happy that her religion works for her, but your experience with churches is that the hype and rhetoric doesn'treally match up to the reality so that you have become like a writer about religion-you would rather just SEE how her faith informs her life rather than being preached or witnessed to about it. Show, don't tell! That will give her an opportunity to be the nice person that she probably is without talking about her religion all the time. Just a thought!
OR,you could just be kind and just let her know that you appreciate good neighbors and while you don't want the witnessing (they consider that "every member is a missionary"), you appreciate her family and community values and share many of them, but you won't be joining any church.
Honestly, I think the first approach would be more successful, plus you will have a great neighbor.