How about those who believe that the said proposition is meaningless because its subject has no meaning. I can say that the proposition "Big Foot exists" is meaningful but its truth value is unknown because it is possible to define (if vaguely) what "Big Foot " is, albeit proofs of its existence remains elusive. Perhaps one day that mystery will be solved.
The proposition "Non-human civilization exists" belongs to the same category even if it differs from the previous proposition in that we may never have an effective method of determining its truth value. That is why some people have faith - a belief that the said proposition is true, even though we will most likely never be able to know one waay or the other.
But the proposition "God exists" is different from the other two because it the determination of its truth function is impossible not to the limitation of human knowledge and methodology, but because the proposition itself is meaningless. Unlike "Big Foot" or "Non-human civilization," the word "god" has no meaning at all. We may not have precise definition of "big foot" or "non-human civilization" but we at least can define some empirically meaningful features that the objects designated by the concepts may possess. We cannot do the same about 'god' without running into blatant absurdities. An anthropomorphic creature running the entire universe? This is absurd on its face. Some nebulous "force" or "spirit"? This is replacing one meaningless word with another.
An idealized image of the parental authority figure retained from the childhood? Thatat least makes more sense, but why would such an image have an existence different from that of, say, cartoon characters? In other words, why do not we ask whether Mickey Mouse or Sponge Bob "exist" but we ask that question about God?
This whole issue of theism vs atheism is a ruse - the fact is that the object of this controversy has no meaning. The whole thing is all about emotions and feelings toward authority figures.
Wojtek