They will go to Hell.

Some may argue that this is unjust, but think about it.

God is perfect and without sin. We are sinners who have rebelled against God. The penalty of sin is death. Therefore, God would be perfectly just in condemning us all to Hell. But because God is merciful, he sent his only begotten Son, Jesus Christ, to die on the cross for the penalty of our sin. Those who believe in Jesus Christ to forgive their sins and repent of their sin will be saved.

This is also where the doctrine of predestination comes in. Not all Christian denominations accept this doctrine, in fact, it is mainly only the Reformed churches that do so. The doctrine of predestination states that people are so fallen in sin that only God can make us see the sinfulness in our lives and to seek Him. Simply put, God choose those who he will save before He created the universe.