It's simple enough, and it only forms one response to one argument I've seen from atheists. In many cases, atheists will respond to believers saying, "sure, God could exist; but so could a(n) [implausibly colored][implausible device] orbiting [implausible location in the solar system]. I could just as easily say I believe in [implausible flying thing]."
This argument is advanced sometimes by very rightly respected atheistic scientists. However, it seems to me to have one very obvious flaw:
Firstly, presume that (and I know this assumption is not universally shared) we assign no qualities to God but omniscience, omnipresence, and omnipotence. That is, we assume that God's consciousness is simply present everywhere that there is consciousness, and that rather than being supernatural, God is in fact perfectly natural, and obeys the laws of physics just as we do. (An immediate question arises as to the word "obeys," since God is the First, Last, and Everything in Between Cause in this argument, but we'll leave that for the moment.) This merely posits that consciousness doesn't need a human body to have coherence, but can fly through space, just as a wireless email does. That this is a drastic assumption to make, and that the atheist would have it proved before accepting it, I understand; therefore, it is not in the least problematic to grant that this could not be true. But it is a step further to claim that it is highly improbable that it's true.
If anything in this definition is necessarily inconsistent with the laws of physics, and if disembodied consciousness has been proved to be simply impossible, please let me know. I am not a physicist. If it HAS been so disproved, however, I would strongly suggest that atheists make that the centerpiece of their efforts to disprove God, prayer, and the afterlife, for none of these things could possibly exist, without disembodied consciousness. That is the key question, then, for atheists and believers alike, it seems to me.
However, if we grant for the sake of argument that God's omniscience and omnipresence proceed from such a principle of disembodied consciousness, and if it hasn't been shown to contradict the laws of physics, then there is a problem with declaring God implausible simply by comparing God to something implausible.
That is that all one need do is compare God to something unseen but plausible, and voila: God exists again. Thanks for your thoughts.