One of the problems with claiming that something is obviously true is that it’s basically a non-starter. That is, just because you saying something is just obviously the case, that does not mean that it really is.
But what is the claim being made? Saying that something is obvious must mean that it is obvious to someone. Well, if it seems obvious to you that God exists, then good for you (I guess). But when you then tell someone that this is the case, from a non-theist’s perspective, this is like telling them that it’s obvious that aliens exist (or that it’s obvious that Obama is still President). Just think what it would be like if someone came up to you and said it’s obvious that Thor exists-just look at the lightening!
Even if God exists, his existence isn’t as obvious as the nose on your face. Thus, I find it weird/implausible that a good chunk of Americans are absolutely certain that God exists. Heck, I’m not even absolutely certain that I’m not dreaming right now! It should also be noted that being absolutely certain of some claim is generally (if not always) irrational; you’re tactility consenting to the fact that nothing would/should change your mind.
All in all, I just find some theist’s attitude on this to be ridiculous. They act like agnostics and atheists are questioning whether water exists.