If an atheist exists, then God must also exist. To understand this maddening and confusing statement we must first examine whether God exists. If he exists then we can breathe a sigh of relief and conclude that atheists also exist.
Whether God exists or not depends on whether anything exists. The question is not whether God exists, but the more fundamental question of “why is there anything at all?” We see that there are things that exist. There are trees and animals and human beings and rock and plants and there are atheists too. But do they really exist? Yes and no. They are here now, but they will not be here forever. All the things we see (including the atheists) are mutable. They change. The tree you see here now will probably not exist a thousand years from now. The atheist you see here and now will not be here seventy five years from now. He may not be here five minutes from now should he have an unfortunate encounter with a bus. In the long view, all the things that exist around us are ephemeral. They will eventually cease to exist. They will be no more. They will be dust. They will be deceased. Since they will one day be dust, how can we say that they really do exist at all?
If they exist–even for a time–then there must be some other quality to their existence that enables them to exist. Something must be holding them up. What is it that causes them to exist even for the short time of their lives? You might call this mysterious something “Life”. To include even inanimate things the philosophers have termed it ‘essence’ which is distinct from ‘existence’. Essence is the fact of a thing’s existence. Essence is the fact of the thing’s “Being.” Since all the things we see–like atheists–are mutable we conclude that they cannot be the source of this ‘essence’. There must be some greater source for “Being” which is not mutable, and does not cease to exist, for that which ceases to exist cannot itself be the source of existence. The Christian mystics call this the “ground of all Being”. The philosopher Thomas Aquinas calls it ipsum esse substantens. According to the Jewish religious tradition this “Ground of All Being” revealed himself to Moses in the story in the Old Testament with the Hebrew name “Jahweh” or the Tetragrammaton JHVH–the translation of the sacred name is “I AM”.
Christians therefore conclude that because atheists exist God also exists.
I am happy to accept that those who cannot or will not accept the existence of God may debate this philosophical explanation. They’ll probably bluster about ‘logical fallacies’ and so forth. That’s fine. Furthermore, I’m happy to concede that it is simply a philosophical explanation. It’s not a watertight “proof” for God’s existence. Why would God provide a binding proof of his existence? That would be to force people to believe in him. He’s not a bully. Besides, there’s no love in that. So this is not a “proof”, but it is evidence (like the other “proofs”) that belief in God is not unreasonable.
This “Ground of All Being” is not the end of the story, but the beginning. Whether this YHVH-Yahweh is a personal God, whether he answers prayer and does miracles and is in fact a Trinity of Three Persons united in one Godhead and whether the second person of the Trinity became incarnate of the Virgin Mary and died on the cross to redeem the human race–all that is another matter entirely.