I pose this challenge to believers: What would it take for you to stop believing in God?
This is an interesting challenge. Why? Well, it may sound paradoxical, but in order for any claim to be true, it must be falsifiable. Further I can meet the challenge from the opposite side. I would stop being an atheist if God were to show himself. Believers, however claim that their is a God - fine. But as rationalists we need to always remain open-minded to the possiblity that we are incorrect. Further if our viewpoint is important to us (as belief systems are) then we need to define them in context.
One of the mentors that I had when I was younger asked me: "Donkey how will you know that you have reached your goals in life?" I responded with something like: "umm ummm ummm". He then proceed to say: "if you cannot define your goals then how will you know that you have met them?" That proved to be massively important in my advancement in life as I was able to just quit drifitng along watching life happen. Why do I cite this story? Because it relates to the principle of falsifiability. For something to be proven true there has to be a point at which we have evidence that it has either succeeded or that it has failed. The same holds true to the claims of a God. There are only two outcomes to the question: "Does God exist?" He either does or he doesn't.
I am not asking you to prove that God exists. (In fact I am not asking you to do anything for me at all - I am just trying to stimulate you to think - and you can ignore me for being annoying if you so choose). I am asking you to look at the rules of evidence and define for yourself the inverse. What would it take to prove to make you stop believing? Its different for each of us actually. Some of us may just try to rationally reason on things, others of us may experience tragedy and then question whre God was and w/or why he would allow such to befall us.
The rule of falsifiability, in short, says that the evidence must matter, and as such it is the first and most important and most fundamental rule of evidential reasoning...For example, the true claim that the life span of human beings is less than 200 years is falsifiable; it would be falsified if a single human being were to live to be 200 years old. Similarly, the true claim that water freezes at 32 F is falsifiable; it would be falsified if water were to freeze at, say, 34 F. Each of these claims is firmly established as scientific "fact," and we do not expect either claim ever to be falsified; however, the point is that either could be. Any claim that could not be falsified would be devoid of any propositional content; that is, it would not be making a factual assertion -- it would instead be making an emotive statement, a declaration of the way the claimant feels about the world. - taken from A Field Guide to Critical Thinking by James Lett