Human beings are remarkably resilient!
We are at the top of the Food Chain for a reason.
Sooner or later the time will come when even the most deeply entrenched person of religious or superstitious character comes face to face with REALITY. It will smack them in the head and get their attention!
When that time comes, all the little seeds of rational doubt you have planted will present the ALTERNATIVE to false and mystical wishful thinking.
Depending on how intellectually honest the person is--the right choice will be much easier to make.
Think of an Alcoholic. You can rant, rave, lecture, threaten and such til you are blue in the face--nothing will matter---UNTIL, the drunk hits rock bottom. That is the moment of reckoning.
If you have offered tough love (but LOVE none-the-less) who do you think will be seen as a faithful ally and a source of nurturing truth unfettered by cultish mind control?
Don't lose hope and optimism, they are the rain which brings forth life and growth and beauty.
Be realistic, steady, encouraging and factual and as surely as day follows night, the clouds will break and the daylight will pour in.