There is none. Neither is there proof God does not exist (depending on how one defines 'God'). So can we all dispense with challenging each other to prove that which is logically impossible to prove?
Instead of proof of God what we need (really) is evidence of God. This would accomplish so much. Theists would have a foundation upon which to argue their beliefs other than "I just know". Atheists would be able to consider the evidence and take it to wherever it leads. For most atheists that would be exciting since most are not anti-God but rather simply not persuaded that a god exists.
What Constitutes Evidence?
Looks like going from proof to evidence does not necessarily make this easier. Have we really thought of what evidence of God might be? How, for example, would we distinguish God from an advanced alien? Is it not possible in this vast universe for a sentient entity to have emerged and grown powerful enough to literally create a solar system, for example?
Well hold onto that thought and let's discuss the easy stuff. When skeptics ask for evidence of God they are (almost always) asking for scientific evidence. That means the evidence would be:
• objective - evaluated and examined by anyone (as opposed to subjective - personal opinion)• empirical - observed and measured• persuasive - sufficient potency to influence opinion
So when a skeptic challenges a theist to show the evidence God exists, the skeptic is asking for objective, empirical and persuasive evidence that God exists.
Evidence of God
With evidence defined we return to what would be evidence of God. First we must define God. With thousands of incompatible definitions for God (and gods in general) our task is immediately hopeless unless we pick a very simple, core definition for God that has as few attributes as possible but still constitutes God. Most religions hold God to be (at least) a sentient entity who created the known universe.
God = sentient creator of the known universe
We now need to determine what would evidence our sentient creator. What is evidence of God?
Sentience is common so that is easily handled. Any intelligent life form can evidence sentience. But creator of the known universe is quite a distinction. We likely could never have evidence that a candidate was THE entity that created the known universe, but would it be evidence if this entity could demonstrate sufficient power to create the universe?
What if this entity, for example, created a new planet in our solar system? Would that be evidence we are in the presence of God? It is beyond our capabilities, but is this really outside of the possibility of a powerful alien?
What then if the entity created a new galaxy? That is such an awesome feat we must be in the presence of God. Or are we being tricked by an illusion produced by a very sophisticated alien?
It seems to me that when one considers what might be evidence of God one can always find a problem. Because God is such a powerful concept there are countless 'feats of strength' that are far beyond us but lower than that of God.
So what is the answer? When we challenge a theist to deliver evidence of God, what kind of evidence would you really accept?
( Everyone is likely to have a very different answer. )