What does the Bible say about why men do not believe in Christ?

In John 10, Jesus explains the reason for unbelief, stating that those who do not believe are not of His sheep. Their lack of belief is not due to insufficient evidence or a lack of willingness, but because they have not been chosen by God. The doctrine of election reveals that believers were ordained to salvation before the foundation of the world, and that faith is a result of God's sovereign grace. This underscores the biblical teaching that not everyone will come to faith, as God has the ultimate authority over who receives mercy and who does not.