How much of the Bible can an unbeliever understand?

An unbeliever can understand only a few basic things, perhaps the story of Bethlehem and some other narratives, but the deep doctrinal aspects of God's Word and end-time prophecies remain a closed book. Scripture says the natural man receives not the things of the Spirit; they are foolishness to him and are spiritually discerned.

Les explains that believers can understand the Scriptures and compare Scripture with Scripture, but the unsaved person cannot see how these things are connected. Paul calls the unsaved person the natural man, who receiveth not the things of the Spirit of God. Les allows that an unbeliever may understand a few things—maybe the story of Bethlehem and some other things—but when it gets down to the deep doctrinal aspects of the Word of God, it is a closed book. End-time prophecies are also a closed book to them. The natural man does not comprehend because he does not want to; it is all foolishness to him. Unsaved neighbors may think something is wrong with someone who attends a weeknight Bible study. Since all of this is foolishness to the unbeliever, he cannot know them because these things are spiritually discerned.

1 Corinthians 2:14

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.

Can unbelievers understand deep Bible doctrine?

No. Paul writes that the natural man, the unsaved person, receiveth not the things of the Spirit of God. An unbeliever may understand a few simple things, such as the story of Bethlehem, but deep doctrinal aspects of the Word and end-time prophecies remain a closed book because these things are spiritually discerned.