There is an argument to be had here, as to whether “existence” is all that one-dimensional. It is easy enough to agree that no amount of deep thinking into an arm chair will yield any information about “the natural world out there”, what ever its bounds are exactly. And of course, those who insist that their gods are real enough with respect to the world to be able to interact with it in some way, from creating things on the outset all the way to performing miracles in the here and now, may come to frustration trying to justify a belief about something so presumably tangible and observable without any actual empirical evidence to point that way.
Still, there is merit to pure reason in principle, if not for synthetic matters such as statements about nature. When the mathematician investigates with pure logic whether or not there “exists” a prime integer within some specified range, or a function that satisfies a particular differential equation, they might end up proving or disproving such existence, but at no point does anything other than axioms and definitions feed into their reasoning. Likewise, no amount of putting rulers to lengths will ever prove with an appropriate rigor any theorem of geometry, though it may help make it intuitively believable, and perhaps spark an idea as to how a rigorous proof could be constructed.
This would of course be a different kind of “existence”, so much so, that one is tempted to suggest calling one or the other by some other name altogether, to avoid the inevitable equivocation. Alas, tradition has it that we do use much the same vocabulary for both kinds of existence, and none of us will do ourselves any favours by adopting an alternative vernacular just to avoid a mistake so easily corrected.
Point is, it depends on exactly what is at issue. The “greatest of all conceivable things”, or the “ground of all being” is of course not the fatherly ghost who magics puppies and bunnies out of his sheer will, or the bodies of saints out of their graves, but still I think one ought to be careful dismissing arguments about the existence of things which are rooted in logic alone on the grounds that (other kinds of) evidence alone can ever substantiate “existence”. That term can mean several things, and there is an entire class of things whose “existence”, for lack of a better word for it, can in fact only be truly demonstrated by logical argument and not by any amount of (empirical) data. These over-philosophized, irrecognizably abstract descriptions of God would, I am willing to grant, fall into that category, and at that point the arguments (at least until they assert any existence beyond the merely abstract, of course) may need to be parsed for their merit, rather than dismissed as unfit for purpose as a matter of principle.