What would it take to force a religious fundamentalist to demonstrate the existence of his or her god or gods?
Nothing will do it if it hasn't happened for 2,000 years. If good things happened, it is because of God's blessings. If bad things happen, it is God's wish.
The only thing that can change the religion's grip is get state control. That is how Islam and Christianity spread after being underground and banished from mainstream for ages. We have to spread atheism, not peacefully but but very aggressively. We cannot work on the edges, we must go mainstream. We must support atheist political candidates at all levels - local, state and federal. We must be more funded, vocal like putting up more and bigger anti-Christmas billboards to kick ass. Supporting FFRF by every atheist is very important. They are doing a good job.
By most definitions of many God thingies they are the more powerful than people-monkey-animals. So you cannot logically force something of a lesser power capabilities to give verfication of something that would be above the lessers capabilities.
Maybe using an example of the non-existent flying spaghetti monster sky God that is invisible in the sky. People are on the ground. Noodles might could drop down and slap the pasta out of an illogical atheist on the ground. Whereas, an illogical atheist on the ground cannot do anything to force the flying noodles to appear so everyone could see them in the sky. If pasta in the sky with meatballs doesn't want to appear, there is nothing anyone can do about it.
What would it take to force them??? Yikes! This isn't a fun torture thread, is it?
Do you mean "what proof would suffice"?
If you want force but not torture...I dunno...maybe make them pay taxes until God herself grants a waiver? In person - at the IRS office - not in some stupid vision.