“I would believe in God if ___ happened.”
This notion is sometimes raised by skeptics when answering the question of what evidence they would require to affirm that God exists. It seems to go along with the “extraordinary claims require extraordinary evidence” assertion. I have even heard skeptics say “if God spoke audibly to me, then I would believe He exists.” This is, of course, a very sophomoric statement.
If the skeptic hears a voice saying “I am God, believe in me,” they would immediately doubt it was anything other than a natural anomaly. The skeptic's job is to explain away any possible sign of God by means necessary. In the case of an audible voice, perhaps it was a hallucination or some previously unobserved aspect of nature. The voice certainly could not be God because “God exists” is not a live option for the skeptic. If the skeptic will not accept any logical or metaphysical demonstration that God exists, they will not accept anything in the way of a sign or wonder.
It is only if a metaphysical demonstration of God’s existence is at least possible that the skeptic could even have something by which to reconcile an audible voice from the heavens (or whatever other signs they may want). In any event, the reality is that if the skeptic is being genuinely skeptical, there is never enough evidence to demonstrate anything out of their existing paradigm. Asking for a 'sign', an audible sign or a molecule with "made by God' stamped on it would never suffice. That is why the request by the skeptic for God to 'show Himself' is fool's talk.