In my brief career as one of those guys who occasionally enters comment threads (either at sciencey blogs where Christians frequently invite themselves to start evangelising or at atheisty ones where they do exactly the same thing) I’ve been asked this a few times; I’ve also seen it asked of countless other heathens, well-known and otherwise: “What would it take to convince you that God exists?” My usual answer begins with a variation of “a working definition of exactly what we’re talking about would be a good start – then we can discuss the options open to either the god in question or those who take on the burden of evangelising for him.” Frequently, it turns out (surprise, surprise) that it’s the omniscient, omnipotent God of European Christianity (as formulated in the fifth century CE and continually revised since) that’s under discussion – but you could use this response or a variant of it for just about any theistic god purported to exist.
If the god in question is, in fact, omniscient, then that god knows exactly what it would take to convince me that he exists without – and this is important, because proof apparently denies faith – negating or harming my free will in any way. He’d know, for example, that taking even mundane, trivial claims purely on faith isn’t something I do lightly, let alone claims that an omni-everything universe-creator cares, for example, what I eat or who I sleep with (or that he doesn’t care at all – this god’s interpreters confuse more than they clarify, which is an observation an omni-God would know that I’ve made many times and which counts greatly against the veracity and power of the various scriptures attributed to him). In fact (and an omni-God should know this all too well), the vague, contradictory, anachronistic and often brutal nature of the scriptures affects my “free will” anyway – by convincing me that they’re simply not reliable sources of either fact or moral guidance.
The fact that I’m still not convinced God exists tells me that God can’t do it, won’t do it, hasn’t done it (presumably for his own reasons), or that God isn’t there – either in any form that can affect the universe or communicate with me, or in any form at all.
But I could be convinced. In theory.