I have have quite a few friends who are agnostics, and I believe that if they do not come to know the Lord before they die, then they will not go to Heaven. I say that 'cause it says in the Bible (I forgot where, but I know it's in there): "No one can get to the Father excpet through me." That means that no one can get into Heaven unless they have a personal relationship with Jesus Christ. Agnostics (people who do not deny that there is a God like atheists do, but they don't accept Jesus as Lord either) do not have that relationship with Jesus Christ, so therefore, yes, they would go to Hell when they died, unless they accepted Jesus before that. You can't mess with God's Word.