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When a Christian asks, "What would it take for you to believe," he or she is really asking, "What would it take for you to believe in the particular god in which I believe?" Many atheists miss this, and skip what I consider to be one of the most important parts of any atheist-theist dialogue: requiring the believer to define his or her god in a coherent manner. After all, my requirements for what I would need in order to believe could vary as a function of what sort of god I was being asked to believe. By skipping this vital step, the atheist guarantees that misunderstanding will result sooner or later. Do not assume that you know what the Christian is asking about. Thus, the first requirement that must be met for me to believe can be stated as follows:
1. I require a clear, unambiguous statement of precisely what the Christian is asking me to believe.
If the Christian wants to know what it would take for me to believe in "god," make this part of the question and give me a precise definition of this god. Which god are you asking about? It is not enough to simple say "the Christian god" or "the god of the bible." Images of the Christian god range from a loving entity which is personally involved in the affairs of humankind to a wrathful, jealous god who kills regularly over minor offenses. It should also be noted that there are many versions of the Christian bible. Are we talking about the New Testament god exclusively and pretending the Old Testament god is irrelevant? If you really want to know what it would take for me to believe, we must first agree on exactly what it is I am being asked to believe. What attributes does this god have? Is this god omniscient, evil, powerful, what? In short, your question is unanswerable unless you define this god clearly enough that we are talking about the same thing.
Some Christians will protest that it is not possible to define their god or that their god is "unknowable." Such responses render the question moot. When encountering such a Christian, the atheist is advised to point out the absurdity of asking someone what it would take for them to believe in something unknowable and without definition. A Christian who claims to believe in something without definition is claiming to believe in a meaningless nothingness, and I see little that would compel me to join them in such a belief.
Assuming we get through the first requirement, and this is unlikely considering that many Christians seem to know fairly little about what they claim to believe, we come to the second requirement:
2. The god articulated above must be logically coherent.
If the Christian has met the first requirement, we have a clear statement of what "god" is and is not. We have a list of god's essential properties. Now we can move to the task of making sure that these properties do not contradict one another. The god in which we are being asked about cannot be a logical impossibility like a square circle. If our god is supposed to be omniscient, how is this reconciled with free will? If our god is supposed to be both all powerful and perfectly good, we consider the problem of evil. Only after eliminating the possibility of logical contradiction can we move forward.
Part of what we're doing here is figuring out the scope of the theistic burden of proof. If I am being asked what it would take for me to believe in a god named Norman who lives in Nashville, TN, and earns a living by repairing golf carts, the Christian's burden of proof might be relatively small. A trip to Nashville might end up being all that is necessary. On the other hand, asking me to believe in a supernatural entity who exists outside of time and space but somehow continues to meddle in human affairs, killed his son (who was somehow a part of him), etc. will have a massive proof burden. To paraphrase Carl Sagan's famous statement, extraordinary claims do indeed require extraordinary evidence.
3. I require evidence to support the belief claim, proportional to the extraordinary nature of the claim.
Many atheists like to jump directly to this step. Given that the Christian has no evidence for his or her belief and not all atheists are sufficiently familiar with philosophy to grasp the importance of the first two steps, this is understandable. Still, without the first two steps, we typically end up talking about two very different sorts of gods and accomplish little. Only after we've properly clarified the belief claim and made sure it survives rational scrutiny, does it really make sense to consider the evidence.
At this stage, most Christians will attempt to frame the question by asking what sort of evidence I would require to believe in the sort of god we have now defined. I would respond that I require two levels of evidence. First, I want the Christian to provide me with some unambiguous evidence that supports the existence of the god we've been discussing - not just some random god but the one we've defined thus far. I set this burden fairly low for my goal at this stage is simply to provide the Christian with the opportunity to provide some clear evidence that supports the existence of their particular god. I do not expect them to be able to do this, for I have encountered no evidence whatsoever to support a god even remotely resembling the Christian god.
Should this somehow be accomplished, we can entertain the second level. At this point, I'd require clear evidence in proportion to the belief claim. Given the extraordinary nature of the typical theistic claim, this is likely to require a minimum of unambiguous "miracles" involving clear violations of natural laws witnessed by multiple witnesses at the same time. But since mass delusions are known to happen from time-to-time, we'd also need independent verification, replication, and so on. The fantastic nature of the claim requires us to set the requirements awfully high here. We'd likely need clear evidence that something was happening that could only be explained by a major violation of the laws of nature. That is, we'd need something that could only be due to the work of this particular god. Merely identifying a phenomenon without a clear scientific explanation would not be even close to sufficient.
What would it take for me to believe in your god? Simple. I'd need a clear definition of your particular god and its essential properties. I'd need to verify that these properties were not in any way logically impossible or contradictory. Finally, I'd need unambiguous evidence in proportion to your claims.
There's only one small problem with this. Any Christian who accomplished it would have destroyed faith in the process. With the sort of evidence required for us to rationally believe in gods, faith would be irrelevant.