What does the Bible say about the natural man's inability to understand spiritual truth?

The Scriptures emphasize that the natural man is spiritually dead and incapable of understanding the things of God without divine intervention. In Isaiah 28:9-13, it is stated that the people have erred and cannot understand doctrine because they have turned away from the Lord. This spiritual blindness is a theme throughout the Bible, establishing that without the work of the Holy Spirit to open their eyes, individuals remain in darkness and cannot grasp the truth of God's revelation. As it is said in 1 Corinthians 2:14, 'But the natural man does not receive the things of the Spirit of God, for they are foolishness to him; nor can he know them, because they are spiritually discerned.'