Most Christians do not really think very deeply about this question. Neither do most atheists. But this question is central to any discussion of whether or not God exists. Most Christians think of God as a person, but who knows everything, can do almost anything and desires only what is good. Most atheists think that the world would not be the way it is now if such a person really does exist. But is that person really what God is?
Atheists and Christians generally tend to agree on what they mean by God. I admit that some atheists either over-simply the concept or misunderstand it, but the for the most part, they are in agreement. The points of this agreement (on the concept of God) are as follows: first, that God is person just like you and me; second, that God knows the moral rules that he ought to follow and finally, that God has desires just like we do. There are other points of agreement as well, but these points are significant for a special reason. Classical Christianity rejects all three of these points. Classical Judaism and Islam rejected these as well.
So which God exists? Is the God that Christians believe in the God of classical Christianity or modern Christianity? More importantly to the debate on the existence of God: is the debate about the existence of classical God or the modern God? I think that it is quite clearly based on the existence of the modern God. So much of modern atheism has nothing at all to say about the classical deity believed in prior to the 18th century.
Let’s describe this classical deity first before showing that atheism has nothing to say about him. First, God is not a person like you or me. He is like a person, but he lacks a psychology in any way similar to ours. He is rational all of the time without exception. He has something like emotions but they never overwhelm him. Furthermore, they are not changeable. Second, God has no moral rules that he ought to follow. Since God is identical with goodness, it is both impossible for him to do evil (or even desire to do so) and everything that he does is good. Third, God’s desires are not similar to ours. It is best to think of God as a being totally unlike us – an alien – than to think of him as another human being. In fact, God is more alien to us than any alien from another planet could possibly be.
Now atheism depends on certain assumptions. If God is like us, then we can look at the world and notice that he does not exist. If we had divine powers, then we would change the world. Therefore, if God is a being like us with divine powers, then he does not exist. But the antecedent to these arguments is the assumption that God is a being like us. This is precisely what classical theism rejects. In fact, I challenge any atheist to come up with an argument for atheism that is actually a problem for the classical theist.
The immediate problem that the classical theist runs into is why we should debate the existence of such a being at all. I have a number of responses to that. First, that is the same being that classical Christians, both in the past and in the present, believe in. If you have no response for them, then they have no reason to take you seriously. (Many of them do not take atheism or modern Christianity very seriously at all.) Second, understanding and responding only to those who think the same way you do is a good way to remain ignorant. Third, without understanding classical theism it is impossible to understand the historical reasons people had for believing in God.
In the debate over the existence of God, the first and most important question is which God are you talking about?