It's often said that an unfalsifiable theory is a useless theory. The best way, in my mind, to determine whether a theory is falsifiable or not is to ask what sort of evidence would convince its believers to change their minds.
I can conceive of no evidence to show that God does not exist. God is unfalsifiable. I can't disprove God, and that's its biggest weakness.
My beliefs, on the other hand, are falsifiable and I'll explain how.
All of the following must stand up to scrutiny in order to prove that a "natural" explanation is not plausible or possible.
If two civilizations with no contact (first, second, third or whatever hand) managed to produce identical holy texts of such length that happenstance would be impossible, that would be significant evidence to contradict my belief that there is no contact between mortals and the divine.
If any holy text produced knowledge that was beyond the possible understanding of the mortal authors, that would also be relatively convincing. For instance, if the Bible made mention of Strangelets, that would certainly shake me. As it is, the Bible gets the measurement π wrong. That sounds a lot more like a bunch of guys in the desert making their best guesses than it does the creator of the universe.
One thing that holy texts like to use is prophecy, and a legitimate prophecy would indeed convince me. Unfortunately, it needs to be very specific and verified. They aren't very good at that. I won't be convinced by the first part of the book making a prophecy about the second part of the book. That's called foreshadowing. Most books have it.
In all seriousness, let's say I wrote the first half of a fictional series with significant statements and foreshadowing about the second half of the series. I die and a fan of the series takes over. How likely is it for the new author to make sure the promises of the first half come true?
I will also require a 100% accuracy rate. I think that's fair to ask of omniscience.
Finding a ghost would be a significant step toward proving the existence of God. If we could find a ghost, a real ghost, we can learn about it. If it is visible, then it interacts with light and can be measured. If it makes sound, then its movement causes vibrations and can be measured. If it can be felt, you get the point. If we can measure the essence of a ghost, we can determine what, exactly, it is. If the ghost retains any intelligence, that is proof of an afterlife. If the same "ghost substance" can be discovered and measured in humans, that is proof of a soul.
If the claims of the religious are true, at least one of these should be producible. Please inform me if any of these happen. Once again, they must all stand up to intense scrutiny. If they are legitimate, they will only be reinforced by the effort.