Certainty seems to be the sticking point here.
How are you defining certitude? Most atheists I know aren't certain of anything, deity included, but they see no reason in the universe for the existence of some supernatural force. Most definitions of God would be "supernatural", correct?
I'm not trying to tell you anything about your intellectual honesty, I'm simply stating how most scientific minds I've interacted with work. I also think your definition of atheism is incorrect.
Science will always have unknowns, but I know of no serious scientist who follows the reasoning of - we don't know, therefore God.
We must be dealing with different atheist, because I know several who are quite certain, and boy a certain someone on here comes across quite vociferously about religion, and it wouldn't be unfair to mistake that for certainty either, although he hasn't specifically addressed it so I won't speak for him.
As for a definition of atheism, depends on who you ask. Some argue as you have that all atheism is is an absence of belief. These are the same folks who say since agnostics don't have belief either, they are really just atheists. That is not the only definition:
The Oxford Dictionary of Philosophy (2008 ed.). Oxford: Oxford University Press. Retrieved 2013-11-21. Either the lack of belief that there exists a god, or the belief that there exists none. Sometimes thought itself to be more dogmatic than mere agnosticism, although atheists retort that everyone is an atheist about most gods, so they merely advance one step further.
"Harvey, Van A.
Agnosticism and Atheism, in
Flynn 2007, p. 35: "The terms
ATHEISM and
AGNOSTICISM lend themselves to two different definitions. The first takes the privative
a both before the Greek
theos (divinity) and
gnosis (to know) to mean that atheism is simply the absence of belief in the gods and agnosticism is simply lack of knowledge of some specified subject matter. The second definition takes atheism to mean the explicit denial of the existence of gods and agnosticism as the position of someone who, because the existence of gods is unknowable, suspends judgment regarding them ... The first is the more inclusive and recognizes only two alternatives: Either one believes in the gods or one does not. Consequently, there is no third alternative, as those who call themselves agnostics sometimes claim. Insofar as they lack belief, they are really atheists. Moreover, since absence of belief is the cognitive position in which everyone is born, the burden of proof falls on those who advocate religious belief. The proponents of the second definition, by contrast, regard the first definition as too broad because it includes uninformed children along with aggressive and explicit atheists. Consequently, it is unlikely that the public will adopt it."
Touching on that last point, yes there are "aggressive and explicit" atheists out there. Theism is active belief, atheism is active disbelief, and agnosticism is, to keep the analogy going actively bowing out because the question isn't one to which true knowledge can be gained one way or the other. If you want to split it up more, some folks talk about hard atheists and soft atheists.
I have no idea the point of your last point. I never said "we don't know, therefore God" That's not an agnostic position. We'd stop at "We don't know" although I suppose we could add, "and we likely never will."
Supernatural is simply something that exists above and beyond natural laws. A sufficiently advanced being could, one assumes, "violate" the laws of physics as we know them. Now, I suppose that wouldn't really be violating the laws of physics as much as having such a better understanding of them that they know all of the ways to get around them...but I go back to the analogy of the ability to create a universe and if you can fine tune it enough and know all of the laws and how they work, you are pretty much functioning as a "God" in that universe. So I suppose it also boils down to, what is the definition of a "God."
Do I think any of the human definitions are likely to be true? No, of course not. I don't think any of the human religions are anything other than man-made. Of course, that has nothing to do with whether they have "no utility" which seems like the much, much more important point that is being missed here over a very long sojourn into definitional nuances.