This is interesting... before I start, let me just say that my "official" belief is that none of us knows the truth of any spiritual realm that does or does not exist - least of all myself.
An apple - each of us who have experienced an apple would relate, more or less, the same details of our understanding of an apple. Sweet, juicy with a firm outer skin - watch out for the funky bits near the core. In that way, an apple can be, sort of, quantified. Not perfectly, of course, but perhaps to within a 90% confidence that people will describe mostly the same things when exhausting their knowledge/understanding of an apple - but probably not exact.
To an even sharper degree, I feel that each and every one of us has a different conception of God. If we were all to "define" God, none of us would give the same answer. Similar, perhaps, in some cases, but very different on at least some levels from person to person. Some believe though they have not "heard" or "seen" God - others claim that they have. Some believe He should be feared in a reverent way, others believe He is made of love. Some do not believe at all - and for wide and varying reasons on that score as well.
From that perspective, I don't believe any of us even truly believe in the same God. We are all "trapped" within our own minds, and restricted to interpretations of our own circumstances and sensory inputs - how can we truly "share" a single concept of God? Especially when (even at 90%) we don't necessarily share the same concept of an "apple"?
My ultimate belief is that, as God cannot be specifically quantified to any degree of confidence - all believers (to whatever degree, however minute) disbelieve the God of every other person, and believe only in the one they have built for themselves.