In Atheism: The Case Against God, George Smith writes:
Defining the concept of god is not an optional chore to be undertaken at the theistís convenience. It is a necessary prerequisite for intelligibility. Assuming that the theist does not believe his theism to be nonsense, he has the responsibility of explaining the content of his belief. Failing this, to state that ďgod existsĒ is to communicate nothing at all; it is as if nothing has been said.
It is obviously the theistís responsibility to define what they mean by god since they are the ones claiming that some sort of god exists. Unless they can provide an adequate definition, then as Smith says their claims arenít really communicating anything at all ó and, in that case, the atheistís position of disbelief is justified. How can it not be reasonable to not believe in the existence of something which claimants canít even define?
This is not to say, however, that atheists have no responsibility whatsoever in the matter. When any debate or discussion about the existence of god starts, itís their responsibility to make sure that the claimants at least try to define what they mean by the term. Itís easy for theists to forget if they are accustomed to talking with other believers where everyone assumes that everyone means basically the same thing (which may or may not be justified ó their failure to explore that question is problematic).
Atheists shouldnít make any assumptions on this score for a couple of reasons. First, it may be that the theist really doesnít have a firm grasp of the concept in the first place; if thatís the case, then any further discussion is pointless. You wonít get anywhere debating Ďgodí with someone who doesnít know what they are talking about.
Second, if you donít establish at the outset what qualities Ďgodí has, then itís easy for the believer to keep shifting the definition later on in order to evade challenges and questions. Make it clear, for example, what is meant by ďomnipotentĒ so that the meaning of this quality canít be changed in order to avoid contradictions with other qualities.