With fear of being preachy -
Expressing 100% certainty about a subject can sometimes be a helpfull approximation, but like all approximation its not true in details, and when used wrongly it can be the ultimate logical fallacy.
More precicely: When expressing 100% certainty (from hereon just certainty) about an issue, its the same as saying:
"No matter the evidence i am presented to, I wont ever change my mind, period"
Or when used on your opponent, it is the same as saying:
"No matter the evidence, even if I can demonstrate all the evidence you based your conclusion on to be wrong, i think your so interlectually dishonest you wont ever change your mind"
and if belief mean that one is certain, belief become sononymous with an even worse fallacy:
"The conclusion cannot be reached in a plausible way. I wont change my mind no matter the evidence, period."
I guess the above conclusion is obvious to allmost all, but i think its wrong to call it agnostisism. Its just plain old plausible reasoning, and the opposite is never practiced, even by those who claim it. I mean, imagine the atheist who die, go to heaven, and after a thousand years with God himself he is as immoveably certain he is not in heaven and God does not exist as he was at the last Richard Dawkins book signing.
Thats why i think its wrong to involve "absolute certainty" in the definition of anything, theism, atheism, whatever.
First off, the definition will cover exactly 0 people, ever. Secondly, its an insult.
Thats not the objective of language.
PROOF a man who is certain about something wont ever change his mind.
Let G be the hypothesis ”there is a god” and "E" be the evidence from a thousand years in heaven, having tea with a person who look and behave exactly the way God is expected to, and perform miracles every day.
We assume: P(G) = 0 and want to calculate P(G | E), ie. How likely the existence of God is given the evidence from observations E. Its a big fat zero:
P(G | E) = P(E | G)P(G) / P(E) = 0.