I believe that he gives us "free will" to decide therefore he cannot know exactly what we will choose.
I am not trying to be the poster boy for Calvinism, but...
Is sin God's will? No, it is the result of our free will. Did God know we would sin given free will? Yes. Did he learn of sin when the forbidden fruit was eaten? No, He cannot learn because He is all knowing.
We have free will...no argument...but your statement above insinuates that God, all powerful God, has no power over your stated free will, not even the power to prompt you to follow Him. Therefore, Christ does not have the power to save you by himself, he needs you to do it for him by choosing Him on your own..i.e., he made an imperfect sacrifice.
Questions for ya'll: I think we agree that Hell is eternal separation from God and as described in the Bible, swimming in a lake of fire for eternity. You are in essence paying for you sins as Christ paid for the sins of Christians. If Christ died for everybody and thus paid for the sins of everybody, then do those who don't go to Heaven actually burn for eternity, or just enter some state of equlibrium not in heaven, or just vanish? If he died for everybody, why would anybody have to pay for their own sins? Are your sins then paid for twice (obviously we could never do enough to pay for our own sins)?