Ok, so Christians state that God gives humans Free Will. Also, God is Omnipotent. Omnipotent means God knows "everything;" the past, the present, and the future. So, God knows exactly what choices a person will make, knows what sins they will partake in, and in a sense, the person's judgment is already decided before they are even born. A person's fate is sealed, because no matter how you argue it, God knows in his Omnipotence what will happen. People are created to be sent to hell and/or heaven, free will or not.
If this made sense to you, then how do you explain things? God cannot be Omnipotent and we still have Free Will. It is a contradiction. Either God is not all-knowing, or people really don't have a choice in what happens after they die. It is known before they even exist.
Imagine I have a time machine. If I watch you do something, then go back in time and watch you do it again, I know exactly what you will do the second time around. Does this interfere with your ability to choose what you will do? Not at all. You chose what you will do, but I already saw it because I manipulated Time.
Time is what it all comes down to. Time is a construct of God. It is something that we are bound by, but He is not. As such, to God, the past present and future all happen in the same one instant (that's not even a proper way to describe it because an instance doesn't even exist, but it's the best I can do). Everything you choose to do happens at the same moment, so while you had free will the entire time, God is not bound by Time and knows your actions before you do.