A phrase sometimes heard from atheists is that atheists and (say) Christians agree on 99.9% (or what have you) of the gods they don’t believe in, but atheists just add one more.
This statement seems to ignore the kinds of evidence which are often a common cause of theistic world-views – that is, gods or God are theoretical posits to explain partially overlapping phenomena.
For example, where an ancient Greek might say Athena was in operation in inspiring someone to speak in a particularly wise and eloquent way that subjectively felt in retrospect in some sense to originate from an unusual source, a Christian will say the Holy Spirit is in operation.
Or consider non-chance coincidings. An ancient Greek polytheist might say a certain god brought about a particular situation that seemed unlikely given mere chance. Similarly, a Christian monotheist would say God in some way brought it about.
The network of the gods (or a particular god) in a polytheistic framework is posited in order to cover some of the same ground as God in a Christian framework. This is why polytheistic paganism in certain forms is closer to Christianity than many forms of secularism – indeed, why Christianity is in a sense a solution to problems that can be more clearly seen (so the idea goes) within various polytheistic world-views than within certain secular ones.
I.e., polytheists and monotheists may disagree on the nature of reality, but they have a significant agreement over certain (so they think) phenomena that call for explanation.
As an analogy, consider an agravitist, who does not believe in gravity. He might respond to a gravitist – someone who thinks the evidence points towards there being something like gravity – by saying they agree on 99.9% (or what have you) of the theories of gravity (for there have been many of these in the history of the world), but the agravitist has merely taken one step further in not believing in one more kind of gravity.
This is to implicitly misstate the evidential situation, because there are large amounts of common evidence which support different theories of gravity.
This is not to ignore that the Christian God is philosophically or theologically very different from most other gods. For example, in classic, Thomistic Christian theology, say, God is not a being (like, say, Thor) but the ground of being, not a good person but the good, and so on. Yet, this is not because the Christian God is therefore not covering a significant amount of similar evidential ground as various gods. Rather, the theoretical understanding has been radically reconceptualized in, say, Neo-Platonic Christianity, just as certain theories of gravity involve radical reconceptualization while still seeking to explain a significant amount of similar going-ons.
So, there are significant differences between the typical conception of a god or gods in polytheistic ancient Greece, say, and of God in contemporary Christianity. Yet, to think of them as almost arbitrary, discrete units, as the atheist sentiment does, is to show an unfamiliarity with some of the similar phenomena that prompt certain theist conceptions.