No less a philosopher than Kierkegaard said the existence of God cannot be proven, and it seems hubristic even to attempt it; nevertheless, here goes.
It seems to me that when someone says he doesn't believe in God, what he really means is, he doesn't believe in a specific god. For example, he doesn't believe there's a guy with a long white beard sitting on a golden throne, which is somehow perched on a cloud, and that he spends his time telling people to go to hell. Well, that's okay, because I don't believe in that god either. There's lots of gods I don't believe in. For instance, I don't particularly believe in Odin or Zeus or Poseidon.
But without specifying whether God has toenails or what God's opinions are regarding the afterlife or Obamacare, we can have a workable definition if we simply say God is the supreme being, and leave it at that. Surely there is a supreme being, right? At any moment in your life, there is one thing, or principle, or something that takes precedence over all else.
Of course, for different people, the Supreme Being, God, is different things. Who can deny that for some people, the Supreme Being is Money? Or Fame? Or Self? I believe that for Marxists, if any still exist, the Supreme Being is the Future or maybe History. It shouldn't surprise us that people have different gods at different times. There have been times, and I'm not proud to admit it, my Supreme Being, my God, was my own belly. I was so hungry, or thought I was, I set aside all good-will and human-feeling until I could cram some damn grub down my gullet. Nancy and I once knew a teenager in the grip of bulimia. I say "grip" because no one who knew her could help but see she was in the fist of a dark and invisible god.
Maybe that's true of all addictions. Addicts are people who serve gods of heroin or alcohol or meth or whatever. But you don't have to be an addict to acknowledge, by your actions if not your words, that for you there is a Supreme Being. It's like that old Dylan Tune, "You got to serve somebody."
So what is the Supreme Being for an atheist? I've thought about it, and I hope my fellow theists won't be offended, but I believe their Supreme Being must be Truth. That has to be it, right? Being an atheist takes effort; the path of least resistance is believing in a skim-milk kind of way, because everyone says so, and it's no skin off your nose one way or the other. A lot of people believe in God that way, and I have less in common with them than the most adamant nonbeliever on the planet. But an atheist has to think about it. She has to weigh the evidence and accept the answer she comes up with. And being an atheist publicly, even today, entails social friction. Why would anyone go to the bother and inconvenience of being an atheist unless there were something bigger at stake, Truth. The atheist has said, "I will seek the truth and speak the truth even at my own cost." Clearly for them, Truth is the Supreme Being.
And truth to tell, Truth is not such a bad God to have. It's certainly a lot nobler and finer than some other gods I've named. I would be be a nobler and finer person if I served Truth more faithfully myself. But as good a God as Truth is, I think I have a better one: Love. What if John had it right, God is Love? What if when we turn toward Love, we're turning toward God, and when we turn away from Love, we turn away from God? Of course, we may not know as much about Love as we think, just as atheists may not know as much about Truth, but what if, at the end of the day, God is Love, and it was just that simple?
Maybe it is.