I don't know if there is anything you can do. You can make your wishes known, & maybe have someone that you trust speak & stress your position, but if a relative feels that they "just have to" go the religion route, it would be tough to shut them down without causing an incident, which probably isn't either worth it, or what any of the mourners would want. You can only hope for the best, &, if by chance it does come about, be secure in the knowledge that you will have not a single inkling of its occurrence!!!

Funerals are for the living. What should you care? You're either in oblivion or grappling with an afterlife that nobody can or should anticipate. I figure you've got more on your plate than dealing with what your friends and relatives want to do to honor your remains. But if this genuinely bothers you, then as others have stated, a will is the most binding thing you can create to make your wishes known and pick your executor carefully.

I'm curious how "it was made very clear that the deceased did NOT want any religious mumbo jumbo". Maybe a sign at the door: "No fairy tales beyond this point!"