(An aside: Socrates thus gets the best line in this cartoon.)
Today I mention it because of this 7th grader whose teacher insisted that anyone who said that "God is a fact" or that "God is an opinion" was wrong. The only correct answer was that "God is a myth."
Now, the way I was taught the distinction, "fact" and "opinion" were mutually exclusive categories that covered every possible statement. "Fact" meant "a statement that can be proven true or false." Opinion meant every other kind of statement.
Thus, "God is a myth" (in the sense of 'myth' as 'false tale') is either a fact or an opinion. Since the teacher thinks it can be proven correct, she is classifying this statement as a fact. But then she ought to recognize that she has entailed that "God is a fact" is true, since she thinks that God's existence can be proven false -- and a fact is the kind of statement that can be proven true or false.
On the other hand, if "God is a myth" is an opinion because it cannot be proven either way, then "God is a fact" is also an opinion. "God is an opinion" is thus a fact, while "God is a fact" is an opinion. That's fun.
In any case, it's all bad metaphysics. Those who think that they can prove God don't try to prove his existence, but rather his necessity: God's existence is of such a different nature than ours that no one believes that we can understand how God exists, but if God is necessary, then we must accept it though we don't understand it. See Avicenna's Metaphysics of The Healing. Aquinas summarizes the argument (far too briefly to give you the sense of it):
The third way is taken from possibility and necessity, and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be. But it is impossible for these always to exist, for that which is possible not to be at some time is not. Therefore, if everything is possible not to be, then at one time there could have been nothing in existence. Now if this were true, even now there would be nothing in existence, because that which does not exist only begins to exist by something already existing. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence — which is absurd. Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary. But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has been already proved in regard to efficient causes. Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.Now Aquinas describes that as a proof of God's existence, but goes on to note that by "existence" he means something extremely different from the "existence" that you or I have -- in other words, he isn't proving "existence" in the usual sense at all. This point he gets from Avicenna, I think, although frankly he could have drawn it from the Neoplatonists or Parmenides. All of these are dense arguments that require years of work to grapple with effectively.
That's work that the seventh grade teacher is unlikely to have done, which ought to provoke some humility -- except that she is doubtless ignorant enough of the whole set of arguments not to realize that it is work that needs to be done to grapple with the question in front of her. She is still trying to prove or disprove God as if God were to be proven in the same way as your postman.
So is God a fact, or an opinion? Both, of course. How could it be otherwise? All things follow from God, and thus all things must be prefigured in God. God is both provable in Avicenna's sense, and outside what can be thought of as a proof for Kant. And thus it is just as true to say that God is neither, of course: all these concepts of human language are limited, and God is not.