Yes, that's their sincere belief. As I posted elsewhere, I have personal experience with this. I don't doubt my own fundamentalist relatives sincerely believe that God would have intervened and cured my nonagenarian grandmother's cancer if we had only authorized exploratory surgery to find out exactly what course of painful, exhausting treatment had a chance of prolonging her life a tiny little bit. They sincerely believe she would not have died on the operating table. They sincerely believe the agony of recovering from major surgery at 93 would have paid off. But that does not make their beliefs reasonable or accurate, and it does not make their behaviour any less toxic.