The only way to express that knowledge of what is unknown, is to do just that. I'm sure there are many things that we do not know, and the only true unknown is that which we do not know that we do not know. If we can wonder about something that has no real answer, yet it is a question we can form in our minds, then there is already much speculation, and whether justified or not, we can claim that there are answers. We are one race of being, engineered/evolved a certain way, with a certain type of brain. I, personally, believe that there are more than likely many things about this vast universe that we cannot even begin to recognize or comprehend. That is what makes it unknown. To believe for even a moment that our minds have the ability to absorb all of the information that this world has to give us, is nothing short of ignorance fueled by undying egomania.

To deal with your question more directly, is there anything SPECIFICALLY that we cannot claim to know? Absolutely not, for as I've stated, anything that can be speculated, can be answered, or at the very least claimed as such. As far as what we do not know on the grounds of it being unknowable by us, I'd love to imagine that the possibilities are limitless, as with our one type of brain, it most definitely is.

What was the purpose of Christ's sacrifice? If I grant you the existence of God, and only take into account the specific details of Christianity, then what was it about mankind that called for such a sacrifice to be made? As human beings under the ultimate deity, we were given a set of instructions, and some would argue even a subconscious in-born awareness of right and wrong. If some of us are unfit to follow these instructions, are we not receiving our just reward in eternal punishment? What makes this Christian God so merciful, as to give us a scapegoat that will assist us in reaching the place of Paradise, when we are all, as any religious scholar will teach us, so short in the standing of the glory of God?

(In fear of sounding disrespectful, I will add that I am not knocking you or your beliefs. Just curious as to what your answer will be. I do respect what you're doing with this debate, and will treat your answers with the consideration they deserve. Thank you.)

I believe we put too much importance on age as a number. The young and old alike may be just as ignorant on certain issues depending on their personal situations. I personally believe that if you want voting done right, it should not be a question of age as much as a question of how the topics at hand personally affect your situation. In my personal opinion, you should be granted the ability to vote once you've either filed for taxes, or have a place of residence under your name (be it owning a home, renting an apartment, or any likewise situation). These are the people that will understand more than anyone what exactly certain topics deal with, and how they should be decided.