Just think of all the "scientific" theories in all of human history that have died horrible deaths in the light of new discoveries. The positions were always held with great confidence right up to the moment they had to be abandoned...and sometimes even afterwards. One new fact, or one new set of facts, provoked a paradigm-shift, however eventual and reluctant.
So, how many facts are there, in the universe, total? More than ten? More than a trillion? More than ten decazillion, cubed? Of course, we could never even guess the number — let alone their nature — of all facts.
That being the case, who can say with certitude that one fact, existing only ten miles under the surface of the far side of Neptune, and only within an eight-inch radius, would not change everything we think we know about... any given subject? One can scoff, he can dismiss, he can bluff... but he can't answer that question. He cannot honestly say that he knows for a certainty, one way or the other, that some fact not yet in evidence would not constitute a transformative, revolutionary revelation.
Yet nobody lives with such uncertainties. Nobody speaks exclusively in the subjective mood. We love the indicative, even more than we should.
So we announce that (say) evolution is an undeniable fact, that the world is X-zillion years old, that homosexuality is not a chosen behavior, that the unborn are not human, that this or that is right or wrong. We speak as if from a perspective of not only omniscience, but omnisapience; as if we both possessed and understood all facts... even though neither is true.
Yet someone has to keep pointing out the emperor's illusory garb: unless the speaker has an infinite grasp of both the identity and the meaning/significance of every last fact in the universe, he has no right to speak with certainty.
Yet the unbeliever regularly does so speak. He does not possess omniscience. He merely feigns it. His intent is to cow opposition (and quiet his own conscience [Rom. 1:18ff.]) by a show of bravado. As we have seen, the tactic often works in the short run.
A second idea lurks under the surface: "Thus Christians alone not only can be, but are obliged to be, humbly certain." The Christian, insofar as he actually practices the faith he professes, necessarily affirms the inerrancy of Scripture as the very word of God. In so doing, he claims to possess a revelation from the only one who actually does know and understand absolutely everything that exists, since He is the Creator of absolutely everything that exists.
Ironically, however, there are those who (A) claim to be Christian, but (B) choose to feign uncertainty on unpopular issues where the Bible is pretty clear.
These are not murky penumbras, but clear doctrines. Not that a devoted opponent cannot fabricate some murk; it is axiomatic that great distance from the Word necessarily creates greater murkiness (Isa. 8:20). Any clear statement can be smudged... including this one. But the professed believer who adopts a pose of tentativeness on such issues is in the precise-reverse position of the unbeliever who adopts the pose of certitude.
In sum: the person who denies God's revelation is obliged to speak uncertainly about everything; the person who affirms God's revelation is obliged to speak certainly about some things (Amos 3:8; Acts 4:19-20; 5:29; 1 Cor. 9:16).