What goes on between consenting adults in private really is no one's business except the individuals involved. This concept should extend to religion as well as other activities. If people keep their religion private, that privacy should be respected.
The trouble, of course, is that believers don't keep their religion private. They are quite eager to put on a public display and even to force others to abide by it or pretend to believe it.
As I mentioned in an earlier post, the burden of proof generally will fall on the party making an assertion of fact to prove that the assertion is true. This is not just a rule of law but of logic and even civilization.
If you are speaking to a believer who, like almost all of them, wants to make his or her religion part of your government, then you have the right to demand that the believer produce proof that his or her beliefs are facts and not delusions.
"If you want to make your religion part of my government, then I have a right to demand that you prove it's true."
"If you want to make your religion part of my government, then you are the one making the assertion that something is true and have to prove it."
This is similar to a point I made in an earlier post about faith in that it makes it clear that the believer ought to have the burden of proof, but this time it isn't a question of the believer simply being logically perverse and childish. This time, the believer has really put his foot in it. He has effectively declared not only that his beliefs are true facts but that everyone in the society should be forced by the government to treat them as such.