There are 3 options and you've already been given all three. 1)Ignore it. 2)write back explaining all the reasons you don't feel you can be part of this false religion. 3)write an appreciate letter back not mentioning anything incriminating.
The only bad choice in number 2 because the mother will take it straight to the elders & that is evidence against you.
I'd say there's a good case for ignoring it if you've only spoken to her twice as its not a letter born of genuine feeling for you. Its a cult response in the sense that anyone leaving the group upsets ones peace of mind. But if you do feel the need, maybe choose a very small thank you card - very small lol, and simply thank them for their thoughts and how much you appreciate it. Don't attempt to explain - its not her business. Maybe quote an appropriate scripture about love or something - that could go down well.
All the best Phoebe