Instead of being honest with your mother, start asking her questions that make her be dishonest with you to defend her choice to stay with a failed doomsday cult.Don't tell her anything she shouldn't already know from her own church's rantings and historical facts she already knows, just ask sincere questions that will affect her .... not you .... her, then keep her honest if she tries any tricks to weasel her way out of an honest answer. Act offended if she tries to fool you into changes of subject or any type of bullying.
Do not express your own opinions, as she has been programmed not to listen to anything opposing, just corner her into defending her opinions and choices, while you practice a bit of shut up.
Be the last to leave, not the first. We all know how that goes.