I just heard a quote where RC Sproul speaks of "absolute certainty about anything" and that this requires omniscience. I disagree. But first, let's make clear that I think he's right about everything else a Presbyterian can get right. (I'm a Baptist and Presbyterians sprinkle and I don't hold that against him.) Moreover, we must recall that Dr. Sproul is not a mathematician, but a theologian. And he's betrayed his ignorance of mathematics in the past.
Let's consider the claim that one must have omniscience to have absolute certainty about anything through the contrapositive. Consider the set of propositions known to omniscience. Every proposition therein will be absolutely certain, by the definition of omniscience. Now consider any finite subset of propositions drawn from the omniscience set. Each element of this finite subset will not be made less certain by this process.
For Dr. Sproul to claim that no one can have absolute certainty about anything is equivalent to positing the impossibility of the last paragraph's finite subset OR the impossibility of anyone ever possessing any such set. If these things are simultaneously possible, then Dr. Sproul's premise is mistaken. Q.E.D.
This assertion that you need omniscience to know anything with absolute certainty implies that nobody can ever know ANYTHING God knows, because it would serve as a counter-example.
Now, for purposes of illustration consider this example. Omniscience knows all the digits of PI. Let's suppose I possess this proposition: "PI is approximated by 3.14159." This is a remarkably ignorant statement. There is a countably infinite number of digits of PI that are left out of this proposition. HOWEVER, no matter how many digits you add to your knowledge, they cannot conspire to overthrow this proposition.
Consider Pope Benedict XVI's lecture at the University of Regensburg. He asserted that reason must be the basis of talk about God. Islam holds a contradictory notion about God's nature, claiming God's transcendence makes his will beyond our categories. To Islam there's no problem with God giving irrational or evil commands. Conversely, Christianity asserts an image relationship between God and man and that the transcendent deity discloses himself in a fashion comprehensible to mankind.
One might even go so far as to call that disclosure a finite subset of propositions drawn from the omniscience set.