I think that there is some common ground that the skeptic (an atheist, agnostic, or perhaps a New Age spiritualist) and the theist can share in this: that human beings, over the course of human history, have invented numerous concepts of gods. No one need deny that. A point of departure, however, is whether or not the same can be said of all of them, namely, that they all don’t exist. Theists and Christians would obviously disagree with that proposition because a central tenet to holding to general Christianity is that God exists.
Now, the first real limitation of this argument is that it merely states an anthropological fact: human beings have in history, and still do, invent gods and new religious beliefs. However, that is where the argument stops. It provides no logical disproof of any god, God, or religious belief. So, in essence, this isn’t even an argument. But let’s go a little further.
The second objection is that this is a downright terrible criteria for determining the truth of anything. Why? Because it commits the Fallacy of False Association. That is to argue that because “some” concepts are incorrect, therefore all concepts are incorrect.
Take science, for instance. Science’s history is permeated with numerous once-held theories that have long since been rejected/superceded in favour of newer theories believed to better explain observable data within the physical universe. However, no scientist is ever going to argue that we can’t trust or “believe in” say, general relativity or evolutionary theory because of all the other superceded theories such as heliocentrism, maternal impression, caloric theory, Aristotelian physics, and so on. Obviously, we all realize that this isn’t grounds for rejecting the truth of a specific thing whether that be a a scientific, theological, or philosophical belief. So, one ought to do two things as a result: one, outright reject this as a criteria for determining truth, and, two, employ or develop a more rational criterion for determining the truth of a belief.
One way of determining the truth of something is to examine each concept, and weigh the evidence and arguments claimed to be in its favour. I think that the rational person will agree that that is where the real discussion is at, and where it needs to take place.
My last point is that the skeptic is making an argument that he likely doesn’t realize will cut him off at the knees. What do I mean? If the skeptic is willing to challenge the theist on these grounds then the theist is fully in his right to ask the atheist if he has applied the same criterion to his system of philosophical beliefs (whether that is his naturalism, nihilism, or materialism etc.). If he has not done so then he is committing a double standard, which is disingenuous in any context of discussion.
Sometimes the argument is phrased differently. Sometimes it goes, I am paraphrasing, “that we are all basically atheists. The only difference between the Christian theist and the atheist is that the atheist only goes one step, or one god, further.”
This is wholly confused. It is confused in the sense that the atheist fully fails to appreciate the ultimate differences between theism and atheism. To the contrary, the fact that the theist believes in one God is all the difference in the world between the theist and the atheist. Entire philosophies, both godless and driven by belief in God, are predicated on that very basic fact. And if that distinction is missed I would seriously recommend the atheist get himself a dictionary.