As a response to question 758 Nicholas D. Smith said,
"Even the atheist grants that God is that being than which no greater can be conceived. Hence, even for the atheist, God exists at least in the imagination (indeed, the atheist claims that God exists only in the imagination). But things that exist in reality are greater than things that exist only in the imagination. So, if God existed only in the imagination, then God would not be that being than which no greater can be conceived--for we can conceive a greater being: one that existed in reality as well as in the imagination). Hence, as God is that being than which no greater can be conceived, God must exist in reality."
However, in question 26, Mitch Green says,
"Many contemporary philosophers infer from the so-called Paradox of the Stone that omnipotence is not a matter of being able to do anything, but only a matter of *being able to do anything it is possible to do*. That observation suggests another possible insight. Consider the Problem...