No reason justifies disbelief.

[Belacqua]

By what standards do you judge that the burden of proof hasn't been met?

By the consensus of experts in any particular field of study.

That's reasonable. I assume that experts are experts because they have studied the field and don't make up their minds lightly. They have solid justifications for their conclusions. 

So for example, if all the experts in a given field (say, biology) apply the same standards they are likely to reach agreement. In biology, the standards would probably be the usual scientific ones: empirical evidence which is intersubjectively repeatable and quantifiable. 

Now suppose you wanted to apply the same standards to religious claims. Is this appropriate? Experts in the field of philosophy, particularly metaphysics, say it is not. Because since the time of Plato God has been conceived of and argued about as a non-material, even a noetic, thingy. These experts hold that God, if such a thing exists, is not detectable or researchable by scientific standards. And there are elaborate arguments to that effect. 

If someone (say, Mr. Wizard) wanted to argue that only scientific standards should apply to questions about God, he would find that the experts disagree with him. Then if he were serious about proving his point, he would have to show that his standards of judgment are the best ones. He would have to meet some burden of proof to show that the way he is judging is a good way.