Yes, according to the Fair Religions Act of 1922, your reason for leaving must be specific and stated three times. Just kidding; there is no Fair Religions Act. The only authority these guys have over you is what you give them. You can write to them in crayon on a piece of cheese 'I think I'm a deity, and screw you if you think you're getting into my kingdom', and they'd call you a couple of times to check up, and then leave you alone, after telling every person you ever loved that you're as good as dead.
If I had the time over again, I'd just turn my back and never speak to any of them again. I wouldn't tell them why because they don't want to know and won't listen. I wouldn't give them the satisfaction of telling me I'm not good enough for their horrid little paradise. I wouldn't put it in writing, because that gives them something to file away, gossip about, and store in their book of pain for all time. Why play their game?
btw; they're going to contact you a couple of times when they get the letter, just to make sure you're sure. Just say you are and that you never want to hear from them again.