There is a common argument among atheists that 'we are all atheists', in the sense that they just disbelieve in one more god than theists do.
For example, we read:
"I contend that we are both atheists. I just believe in one fewer god than you do. When you understand why you dismiss all the other possible gods, you will understand why I dismiss yours." Stephen F Roberts
Failure of Analogous Arguments
Ed Feser, arguing from philosophical point of view, replies:
While your average “Internet Infidel” seems to regard the “one god further” objection as devastatingly clever, it is in fact embarrassingly inept, a sign of the extreme decadence into which secularist “thought” has fallen in the Age of Dawkins.
Suppose someone skeptical about Euclidean geometry said:
When you understand why you regard all the particular triangles you’ve observed as having sides that are less than perfectly straight, you will understand why I regard Euclidean plane triangles as such to have sides that are less than perfectly straight.Or suppose a critic of Platonism said:
When you understand why you regard the things of ordinary experience as in various ways imperfect or less than fully good instances of their kinds, you will understand why I regard Plato’s Form of the Good as being less than fully good.Would these count as devastating objections to Euclidean geometry and Platonism? Would they serve as fitting mottos for blogs devoted to “Common Sense Anti-Euclideanism” or “Common Sense Anti-Platonism”? Obviously not. They would demonstrate only that the speaker didn’t have the slightest clue what the hell he was talking about. The “one god further” objection is no better than these stupid “objections” would be.
Rebuttal in Theism
According to Theism, God is not just one being among many, but is being itself. Atheists may compare God with other possible creatures such as Santa, Zeus, and the Flying Spaghetti monster, and claim that where is an equal lack of existence for all of them.
However, God is the "I Am", is being itself, and is therefore in a different category. None of the other putative Gods ever claimed, or had it claimed of them, that they were 'being itself'. In philosophy, this is called aseity, because in latin a se means 'in itself' or 'from itself'.
All we need to note now, for our Rebuttal, is this attribute of God. Just from God being "being itself", we prove that there is only one God:
For consider the logical possibility of two Gods. They would both be being itself, since they are Gods. But God is being itself. Therefore they are both God, and hence identical.
So the nature of God as 'being itself' implies monotheism. It implies that there can only be one god. Not many, and not zero. One is a special number when it comes to counting God.This is the reason for monotheism, why Gods more than one are rejected, as also are Gods numbered only zero. Theists disbelieve all God who are not being itself. That set includes Zeus, Santa, the FSM, and all those others that have ever entered into polytheism (except, possibly, just one).
More general discussion is here.