I would go for quitting over being fired. Yes, it might be tricky to resign.
A suggestion would be to use one of their pet phrases on them, "a matter of conscience". No details. They will think you are doing something borderline "a matter of conscience" and you will know it is because you know they are a friggin' cult you don't want to be a part of anymore, again, "a matter of conscience". If they press you for juicy juice just stand firm and ask that they respect your feelings. If a knucklehead comes right out and asks if you've been doing anything "unscriptural" just say No.