George posted a link to a theist website that asks three questions of atheists. I assembled this post in my head before I read George's (and anybody else's) reply, so that it wouldn't colour my own response too much.
Question one: What would it take, or what would have to happen for you to abandon your position of atheism and come to a theistic view; not just an agnostic possibility of God, but an actual belief that a Deity does exist?
Good, reproducible evidence is pretty much the only thing I can request here.
I can't really consider unexplained phenomena as evidence for God, because history has shown us that events humans used to consider as only explainable by premising deities turned out to actually require no divine interference. Unfortunately, the more we seem to examine the universe in detail, it seems that any version of a god, other than the nebulous "prime mover" that originally sparked the creation of the universe, seems completely unnecessary and more and more unlikely.
For the same reasons, I consider miracles to be poor manifestation of a God's existence. Ignorance of the cause of a certain unlikely occasion is not evidence for anything other than... ignorance.
I'd like to say that personal revelation would be nice - maybe the clouds parting, and a booming voice announcing to me that it was the creator of the universe. But I'm too aware of the weaknesses of the human mind, and that such a thing would most likely be a result of a psychotic episode. Otherwise, how could I be sure that my experience was more realistic than that of Mohammed, Joseph Smith, or any run-of-the-mill schizophrenic?
Question two: What would it take for you to believe Christianity is true?
Which version of Christianity? There are over five thousand different sects of Christianity, which makes this question a little too simplistic to answer accurately.
For instance, whatever evidence that might convince me that Jesus was an actual historical figure that was able to come back from the dead after a couple of days of actual death and and decomposition would have to be coupled with a enough evidence to overthrow pretty much all of modern science to show that the universe is less than 10,000 years old and that the myth of Noah's ark actually happened for me to believe in any of the myriad of young earth, biblical literalist Christianities.
Question three: Why would your answers to the above be sufficient to convince you theism is true, and that Christianity was true?
Like gravity, wavelengths of radiation outside of our ability to detect them with our natural senses, and dark matter, it all comes down to the evidence.
As an atheist, I have a natural inclination to require more evidence for religious claims (not just Christian claims, mind you, but from all religions) than might be fair. Because of this, I figure the most reasonable I can be would be to require the same kind of evidence that I'd request before I believe that intelligent aliens are visiting Earth in technologically advanced spaceships.
If a UFO landed on our planet and the inhabitants of the vessel made contact with a large swath of humans, including media and scientists, and gave us a reproducible explanation of their origins, how they arrived on Earth, etc., I would have a difficult time denying the existence of intelligent aliens.
If God wants to visit us, making himself available to the media and scientists, giving us a scientifically valid and confirmable explanation for how the universe works, the origins of life, and human history, then it would also be difficult to deny his existence.
I appreciate what seems to be a non-judgmental request for information from atheists about their beliefs, but these three questions seem to me to be missing one very valuable point: the Christian God is all-powerful and all-knowing. Even if I don't know what would convince me that He exists, Yahweh does, and he supposedly has the power to make me aware of his existence. That He has refused to do so, even though I actively sought Him out for a number of years in my life, is among the best evidence that God doesn't exist. At the very least, He doesn't seem interested in spending eternity with me, and, in return, I couldn't care less about Him.