Without looking it up, what is 73 x 73? Off the top of our heads, most of us won't know the answer. I'm going to tell you now, the answer is 5,329.
Now, is it arrogant of me to assume I know that for a fact? Obviously not. But there are people in this world today who do profess that claiming to know certain things is arrogant. For example, where did we come from? The answer is found in the book of Genesis. But to some people, merely claiming to know that is arrogant. Why? Because we apparently do not know that, and there is more wisdom in endlessly seeking the truth than to actually reach that end, and claim to have reached it.
But there is a difference between "you don't know" and "I don't know". Maybe the Agnostic in particular genuinely does not yet know that Genesis is the true answer to our origins. Does this mean you do not know? Only if the Agnostic is going to claim that he is the standard of all human knowledge! But this is obviously pure arrogance. To assume no one can know an answer to a question, simply because you do not know, is as arrogant as assuming that because you can only run at 10mph, no one could possibly run at 20mph. (For reference, famous runner Usain Bolt has been clocked at nearly 28mph).
It gets even worse when the Agnostic says you can't know because no one can know, as this is still assuming himself as the standard of human knowledge, not only in its current state, but also in all possible states. It's like those who used to claim "if we were meant to fly, God would have given us wings". The existence of the airplane shows how asinine this was.
Furthermore, with the Christian faith in particular, we're not talking about things we may one day discover. In time, we may discover how to enable your average Joe to go on a vacation to the moon, as easily as he may now take a trip to Florida. For now, however, only a handful of people have managed to go there, and this only as recently as the 1960s. But the Christian faith is based on events that happened no more recently than 2,000 years ago.
Now, is it arrogant of me to assume I know the Battle of Hastings took place in 1066? "Yes, we might one day discover that it actually happened in 1845". No, we know it happened in 1066 because of historical records. We may occasionally discover things from the battle in or around the site, but as far as the records go, we're not looking for new information, nor do we need to.
In the same way, there are questions about the Bible we haven't got an answer to. What is the precise location of the Garden of Eden? Don't know, it was destroyed in the flood (2 Peter 3:6). But the Bible is a rather solid record. Even setting divine inspiration aside, a lot of it is the primary or secondary testimony of contemporaries. We've known it was true from the time it was written (when most readers would have been able to verify elements based on what they, themselves, had seen) right up to today, when we have an amazing amount of evidence to back it up. Rejecting it as an answer just because it is an answer is just plain folly.
It also highlights the third option for the Agnostic mindset. There is a difference between "you don't know", "I don't know", but option three is "I don't want to know". Agnostics, at least the type that will typically attack your confidence in your religious beliefs, are rarely so ignorant as to not know who God is. But this knowledge is inconvenient to us. If you know who God is, you know 1. that you are accountable to Him, and 2. that's a problem for us as sinners.
See, we are rebels, because we are descended from the first rebel. Each and every one of us has offended our Creator, and as a result, punishment is due. But none of us can bear that punishment. Thankfully, none of us need to, because the same God to whom we are accountable has provided an alternative.
2,000 years ago, the Son of God came to Earth. He was born as a man, lived a perfect life, and died on a cross. That death was your punishment. If you repent and believe, all of your sins are accounted to Jesus. The alternative is to continue to resist and receive the full punishment yourself. This, my friends, is not worth it.