I have heard several atheists and agnostics say that they are looking for God but haven't found him. I have had quite a difficult time in the past understanding this claim. But then I realized (based on their objections) that they were searching for a god based on what they wanted God to be (their own definition of "God").
If you are looking for something that is not there (does not exist, in this case), you won't find it. Let me put forth an example of what I'm talking about: Let's say that someone asked me to bring them some ketchup; I go look through the refrigerator and come back saying that there is no ketchup. They insist that I go look again. Instead of just looking in the door (where I keep ketchup), I look on the shelves and in all the drawers- nothing. I go back telling them where all I have looked. They insist that they are not out of ketchup. Frustrated with their ignorant insistence, I (smugly and obnoxiously) take them over to the fridge and ask this question: "Do you see anything red, in a tall, plastic container labeled 'k-e-t-c-h-u-p'?! No? Then we agree that you are out of ketchup." (I purposely made that last sentence a statement not a question) They smile and slowly walk me over to the cupboard and show me the short, plastic container that is holding a green substance with the label "catsup".
What was wrong this? I assumed that my friend's idea of ketchup was the same as mine, not to mention the fact that I was looking in the wrong place too. Our definitions were different and we assumed that the other held the same definition. When my friend realized that I was looking for something that did not exist, he showed me what did exist. At that point, it would have been stupid of me to insist that he was still out of ketchup, instead I should change my definition of "ketchup" and acknowledge that I was looking for the wrong thing and in the wrong place.
Obviously, if someone is looking for something that does not exist, they won't find it. There are many "gods" (slight differences in definitions) that do not exist. It is easy to prove something does not exist if that specifically defined "something" truly does not exist. It is easy to demonstrate to and be accepted by others if their definition of that "something" is the same as yours. However, if the definitions are different (assumed, so it is not so obvious), the task is not so easy, until the definitions are clearly stated and agreed upon. If the definitions are not agreed upon, then they need to be debated and settled; otherwise, debating the original issue will not be fruitful.
In most debates with atheists, theists would concede quite a few points if they knew the explicit definition that the atheist was using. Then the theist would say, "...however, that is not the God who's existence I am defending." We all have heard the phrase "know who you're fighting against (your enemy)." If the atheist does not know what he's against, he can't bring appropriate objections to the table. If the theist does not understand what he's debating against, he can't bring appropriate objections either. Any objections that are not directed at the correct understanding of the adversary will be seen at red herrings to avoid the issue and will not be taken seriously. Not knowing and understanding your adversary is a commitment of the strawman fallacy. Maintaining a strawman can lead to the strong temptation to resort to ad-hominem attacks. For more on those, see last years post "This Argument is Full of Crap".
What should we be gaining from this? Well, I have something for both sides of any debate or discussion. First, both sides need to place their specific definitions on the table. Second, both sides need to be able to recognize when they are arguing against something that the other is not defending (strawman), then change to argue against what the other is defending. Otherwise, both sides are just "spinning their tires" and only galvanizing the other's frustrations (bait for ad-hominem attacks).
As a Christian apologist, it is my responsibility to make sure that my definition of "God" is clear. It is also my responsibility to make certain that when I am arguing against another position that I understand what I am arguing against. If my idea of my opposition's idea is incorrect, I need to be humble enough to change my idea to match their's (no matter how much more difficult arguing against the real idea might be).
Of course, when I'm trying to match my ideas to someone else's ideas, I might be tempted to but should not tell them what they believe (different from pointing out implications and inconsistencies). More can be found on this hairy area in my post "Misengaged in Battle".