Every once in a while (more than I'd like to, unfortunately) I hear theists of all stripes claiming that evidence for God is out there. Atheists, apparently, are just too blind/ignorant/closed-minded/stubborn/possessed by the devil to notice it. Some even claim that we DO see the evidence, we just pretend we don't so we can sin.
Obviously, these claims are excuses made in order to duck the immense burden of proof belonging to those who claim belief in the supernatural. However, I'll be friendly for once and I will describe in detail the type of evidence I would need in order to consider a belief in a God.
If every person claiming to have been "divinely inspired" come back with the same messages, revelations and assertions, I might be inclined to think that there might be some truth to their claims. However, the types of divine revelations we see and hear about today are suspiciously close to what we would expect if people were just making stuff up or mistaken. Christians, Muslims AND Jews all claim that their texts are the inspired words of divine beings. Yet all of these "inspired words" say different things. Not only that, but every denomination of Christianity claims (oddly enough, for the same reasons) to be interpreting the Bible correctly while the others are distorted in their thinking. Why does this not seem fishy to most people? It's even gotten to the point where some of these churches actually distort history to make themselves seem like the "original" interpretations of Christianity. Both the Eastern Orthodox Christians and Fundamentalist Christians such as Jack Chick claim that their systems of belief were founded by Jesus Himself. In actuality, Jesus never founded a church. His original followers were all Jews. Christianity developed as a result of both evolution of certain beliefs and the attempts of more powerful believers to wipe out the ideas of the weaker ones. For all of this "divine inspiration", no one can seem to agree what these supposedly divine beings are saying. Either these beings suck at communicating or people are (consciously of unconsciously) making stuff up.
Now if aliens from the andromeda galaxy came to earth and they were undeniably Christian and they instantly recognized we humans as God's chosen species, I would give the religion another thought. Same goes for any other faith. However, all of this as yet to happen and I remain unconvinced.
A Demonstrable Miracle/Act of God
Miracles are yet another proof people claim to have that their deity exists. Yet somehow, every single religion boasts miracles, even though not all of these religions can be correct. Now all we have to do to find that many miracles are either intentional or accidental hoaxes is go to skeptics dictionary and look them up. In order for me to accept an event as miraculous, it would have to be completely unprecedented. The remission of diseases such as cancer can't count because spontaneous remissions are within the expected behavior of the conditions! The classic example of healing amputees comes to mind here. Now keep in mind that a single regenerated limb using prayer or Holy Water isn't enough. The event would have to be able to be tested and repeated under controlled conditions and the conclusions would have to be peer reviewed. This is not any sort of special requirement. This is exactly the process that every hypothesis must go through before we can consider it a truth about how the world works. Note that I include the "creation of the world by design" in this category as well. I do not consider this a miracle anymore than I do the spontaneous remission of cancer and for the same reasons. Besides, the world is amazing and I love it... but if it was "designed", the designer was not very intelligent. Don't tell me to "look at the trees" when I've seen those things fall and cause destruction.
A Sign of Divine Wisdom
This would be similar, but not identical, to the above demand. What I'm evoking here is the tendency for divine revelations to contain the same errors and prejudices as the humans who receive them. You know, how the Bible didn't contain any accurate scientific information and in "science" it did contain was dead wrong? I simply expect a creator of the universe to know how his own world works. I also expect prophecies to me more than just vague predictions that can be ascribed to any event in hindsight. When we find a Holy Book that, out of no where, gives us a detailed explanation of exactly what happened before the Big Bang, for instance, and that explanation turned out to be true, I might be inclined to believe it. Until then, consider me a skeptic.
In all honestly, one or more of these three things is all it would take to get me and many other atheists to lean towards belief. It really is not asking much. Oh yeah, and I refuse to take any of these huge theistic assertions on faith. It's against my principles, no matter how good pretending to know everything feels.