In The Atheists Debater's Handbook, B.C. Johnson writes:
Both the theist and the atheist agree on many points. For example, they share a belief in the existence of a physical universe composed of orderly atomic structures, and they may even hold very similar moral beliefs. However, the theist asserts that a further belief is necessary in order to explain both the existence and the characteristics of those things about which he and the atheist have similar beliefs.
The first point to notice is that the theist has proposed to explain a set of facts. Now if one offers an explanation of something, one must be prepared to provide reasons for accepting the explanation. Consider what would happen if justifications were not required for all proposed explanations. We could “explain” something in whatever manner suited our whim. We could assert, for instance, that the earth turns because muscular ghosts push it. It might be claimed that the wind blows because air spirits are fanning themselves. We could even offer incompatible explanations of the same facts and there would be no way to decide among them.
The atheist is like a man who does not necessarily claim to know what makes the earth turn but who nevertheless does not believe that muscular ghosts push it. It is vital for the one who believes in such muscular ghosts to offer reasons for his belief. By the same token, it is incumbent upon the theist to provide reasons for his belief that God is the true explanation of the universe and morality. The atheist, for his part, does not necessarily offer an explanation; he simply does not accept the theist’s explanation. Therefore, the atheist need only demonstrate that the theist has failed to justify his position.
It’s pathetic, quite frankly, to see theists insisting that unless atheists offer as “complete” of an explanation as “god did it,” then God must be accepted by default. This appears to be the strongest defense they have of their “explanation” and is a sign of just how awful it is. When police claim that the explanation for someone’s death is “murder,” they don’t defend this by saying it’s the job of others to prove the superiority of some alternative. When doctors claim that the explanation for a disease is some behavior, they don’t defend this diagnosis by insisting that it must be accepted unless a better alternative can be offered.
By definition, an explanation must establish causal connections between one event and another. An explanation must expand our knowledge and understanding of the event being explained. The theistic “explanation” if nothing of the sort because it seeks to “explain” something unknown (like the origin of the universe) with something at least as mysterious and, according to theological premises, permanently beyond our understanding.
It’s absurd to offer as an “explanation” something which you turn around and immediately say not only can’t be explained, but can’t even really be understood. Well, if it can’t be understood, then it’s unreasonable to think that we can understand it enough to conclude much of anything about it — especially something so significant as the idea that it could be responsible for the universe around us.