I ask this question a fair bit these days, but sometimes it confuses people. Let me explain the question.
Here, god-claims, and claims-of-miracles both fall under what I am talking about when I say 'supernatural claims' (though it's a much broader question).
1: The world is full of supernatural claims (ghosts exist, fairies exist and are tiny and have wings, fairies exist but are human sized and don't have wings, Thor exists, Zeus exists, Ahura-Mazda exists, the Rainbow Serpent exists, Yahweh exists and is a trinity, Yahweh exists and is not a trinity, Ganesh exists, Jesus is divine, Jesus is a prophet and not divine, Jesus is neither a prophet nor divine, ...)
2: most such claims contradict other claims. Where multiple mutually contradictory claims exist, most of them must be wrong (since at most one of a set of mutually contradictory claims can be true at once). If we aren't especially lucky or careful - unless we're 'special' or taking particular precautions, we have no basis to assert our beliefs are more likely to be right than any others.
Without good reasons that establish otherwise, claiming we're special is special pleading; having suitable precautions requires some way to tell claims apart that reliably distinguishes true claims from false ones.
3: If most supernatural claims of a particular type are necessarily wrong (because of 2:), then without some reliable(1) principle(2) by which to evaluate them, we are left to use unreliable means.
4: if we use unreliable means to evaluate them, then unless we're somehow astoundingly lucky (again, see special pleading), we will end up believing almost entirely false things, on average.
(1) reliable, in this context would include that different people applying the same principles to all available information will either come to consistent conclusions, or will have a clear means by which to reconcile any differences in conclusion - differences will be resolvable by means provided within the principles themselves.
(2) principle, in this context, means a mechanism by which information and ideas and claims can be compared and selected between - such as a general rule by which we can identify either true or false claims. It doesn't include evidence or logical arguments, but could include mechanisms by which to evaluate such things (evidence and logic aren't principles as such, but if we had a principle like 'vaid logical arguments based on true premises will be true' that says that we can rely on logical arguments that fulfill those conditions). We need principles to evaluate reasons rather than just having 'reasons' in the absence of principles by which to judge them, or we again end with with special pleading. That is, our own reasons are held to lead to a valid conclusion but someone else with the same kind of reason but a different conclusion to our preferred one is not. Either all such reasons are valid, or they are not.
A collection of principles should be applicable to all similar claims.
So to get back to the question:
What's a reliable principle by which to award supernatural claims belief?
This raises some interesting aspects to consider as part of the main question, relating to consistency between how people think about religious beliefs held by others:
"What is a reliable principle by which we may judge to be true the supernatural claims of religions other than one you hold?"
"Do you apply the same principles to the supernatural claims of your own religion?"
And finally ... the big consequence of the usual answer (that there aren't actually reliable principles, and hence no actually reliable reasons):
If your reasons are unreliable, why on earth would you rely on them?