Why can believers understand the Bible but unsaved people can't?
Believers can understand the Bible because these things are spiritually discerned; they can compare Scripture with Scripture. The unsaved natural person receives not the things of the Spirit of God, regards them as foolishness, does not want them, and therefore cannot see how they connect, especially deep doctrine and prophecy.
Les explains that believers can understand the Scriptures and compare Scripture with Scripture, but the unsaved person cannot do that; he just cannot see where these things are connected. Paul calls the unsaved person the natural man, who receiveth not the things of the Spirit of God. Les notes that the unbeliever may understand a few things, such as the story of Bethlehem, but when it comes to the deep doctrinal aspects of the Word of God or end-time prophecies, it is a closed book. The natural man does not comprehend because he does not want to; it is all foolishness to him. Since these things are foolishness to the unbeliever, he cannot know them because they are spiritually discerned or understood. Les also illustrates that unsaved neighbors may think something is wrong with you for attending a weeknight Bible study.
But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.