"In this world nothing is certain but death and taxes." *Everyone has heard this old adage, and everyone understands its basic truth. The humor lies in comparing a political evil to a physical one, a contrast between the abstract and debatable, and the concrete and undeniable. My point here is that death is understood by all adults as inevitable; indeed, one could plausibly claim that the dividing line between childhood and maturity is the knowledge of the inevitability of death. If there is to be any meaning to the word "certainty," it must include the certainty of death.
Yet the central thesis of Christianity is the outright rejection of this self-evident truth. The message of the cross is Jesus' triumph over death; the message of His ministry is that death can be defeated. To speak of the Christian god is to implicitly assert that death is not certain. By death I don't mean a brief sleep before the bodily resurrection, or the trivial shedding of our physical bodies; but actual death: the final goodbye, the eternal absence, the hard lesson we all learned when a pet or a relative passed away.
Here, then, is reason enough to justify atheism (at least with respect to Christianity). If we can know anything, we know that people die, and that means we know that Christianity is false. When I say I am certain that God does not exist, what I am really saying is that I am certain that death is real.
Therefore, the existence of God is not a neutral proposition, unweighted by evidence for or against; it is an active denial of what we all know to be true. To make that assertion requires a positive act of faith; to lack that faith is to be functionally indistinguishable from an atheist. There is simply no room for agnosticism with respect to the Christian god. One either believes; or one does not; one either has faith, or one does not. One does not simply not know whether or not people die; rational adults do not lack sufficient information to form an opinion on mortality.
Thus agnosticism, with respect to Christianity, is philosophically untenable, however socially useful (an agnostic is sometimes defined as an atheist who doesn't want to argue about it). The only way to pretend that the claims of Christianity are in a state of uncertainty is to revert to a child-like innocence on the topic of death.
Well, there is another way, which is to redefine terms and conditions until all meaning is lost: but that way lies solipsism.
* Attributed to Ben Franklin, although, ironically, we are not certain it originated with him.