While I am having difficulty finding a reputable source to confirm it, I have been told the following odd thing about human psychology: if a person wears glasses that flip their vision upside down, about three days later their brain will adjust and invert their sight. If they then remove the glasses, their vision will seem to be upside down before it flips again, more quickly.
I don’t think all that many people are aware of this quirk of human psychology. As such, it seems like something you could build a cult around.
You would come up with a long and convincing build-up to a supposedly sacred ritual in which people wear the glasses. You tell them that if their vision eventually flips, it is because your deity has deemed them worthy of being tested. Then, you tell them that when they remove the glasses, one of two things will happen. Either their vision will be inverted forever, or it will flip back. Tell them that if it flips back, it means your deity has found them worthy, and they are on track for some sort of magnificent afterlife.
Because people would think the flipping was supernatural, it would make a gullible subset believe anything else you cared to tell them (like about how they need to sell their home to help fund the crystal statue that will bring about the end of the world). Eventually, people will leave the cult and tell their story, and neurologists will appear on the news to explain that the vision-flipping is normal and being used to scam people. By then, however, you will be long gone with a lot of money.
I think this could work partly because vision is such a key part of a person’s life. Seeing it flip would be a powerful emotional experience, especially if you were prepared in advance to interpret it in a specific metaphysical way. The period between the first and second flip would be full of anxiety – since you already know the flipping is possible, but fear it could be permanent for you. Then, the second flip would really lock at least a few people in. It would feed their narcissism by telling them they are special, and it would seem to be something beyond the power of ordinary science or reasoning to explain.
I think people have probably bought into cults on the basis of less convincing evidence than this. Get a couple of celebrity adherents and the road to wealth and influence would be short and smooth.
Disclaimer: While you might actually be able to start a cult in this way, it wouldn’t be a very nice thing to do.