Certainty, even a false certainty, has a deep attraction for the troubled or lost or otherwise disconnected. Under those circumstances, it really wouldn't take much to get sucked into a religious (or other) social vortex. I hope all goes well for her - the Mormons are not the worst, though I find their attitudes to femininity to be not very wholesome. And they do do a lot a doorknocking etc.

it happens slowly

Ask them about their weird undies!!