first off, it is impossible to completely prove or DISprove the existence of god. also, why must we as imperfect, finite beings be able to define a perfect, infinite being. that is against logic. from what i have always gathered, agnostic was one who believed in the existence, but believed against the ability to know it. i will do more research on that, but that was always my take on it.
and no rationalists do not only believe in what makes sense to them, they believe in what makes sense. period. if they are the former, they are not true rationalists. also, there is no way you can compare the existence of god, and the existence of santa claus, or demons. first of all, i will use some of descartes argument. I have to look it up to put the whole thing in here later, but right now i will focus on the main parts. being that god must be perfect and cannot be to the contrary, we as imperfect beings are logically not able to create an idea of a perfect being. so that idea of a perfect being must of come from somewhere. it of course didn't come from inside our minds, for our imperfect minds logically cannot form an idea of perfection. but its still there. therefore, something must of allowed us to have this idea of a perfect essence. again i will put his whole 13 step argument in here later.
and about santa claus, its easy to understand where santa claus came from... sure, he might not exist, but his characteristics do. we all have an idea of a jolly fat man. a red suit. we have reindeer, and we have things that fly. we can put it together and make up something that is santa claus. however, back to god. there is nothing in the physical world that is perfect or infinite, so where did we get the idea from? Our minds cannot create what doesn't already exist in the world. Yet we are able to create an idea of something that can not be measured in the physical world. so i ask you, where then did the idea come from?