

Unsaved people who hear or read the great spiritual truths from the Bible do not have the capacity to fully understand them. Oftentimes, those truths seem like foolishness to them. Here is how Paul explained why those who are not spiritual (unbelievers who do not have the Spirit of God abiding in them) are unable to understand spiritual things:
When we tell you these things, we do not use words that come from human wisdom. Instead, we speak words given to us by the Spirit, using the Spirit’s words to explain spiritual truths. But people who aren’t spiritual can’t receive these truths from God’s Spirit. It all sounds foolish to them and they can’t understand it, for only those who are spiritual can understand what the Spirit means. Those who are spiritual can evaluate all things, but they themselves cannot be evaluated by others. For, “Who can know the LORD’s thoughts? Who knows enough to teach him?” But we understand these things, for we have the mind of Christ. 1 Corinthians 2:13-16
The Gospel message is one critical spiritual truth that Jesus instructed believers to share freely with those who don’t believe. When we share the gospel with someone, the Spirit of God works in the heart of that person so that they can understand God’s plan of salvation. The Holy Spirit convinces them they are sinners who have broken God’s laws, and enlightens their understanding that Christ died for their sins (took the penalty for their sins), and that through faith in Jesus, their sins will be forgiven and they will live in God’s presence forever. If they receive this message and make the decision to believe and receive God’s plan of salvation and gift of eternal life, the Holy Spirit will come to permanently reside in them and will then help them understand all other spiritual truth. Only God Himself can make our spirit alive so that we can understand spiritual things and relate to Him. Verses 19-24 of 1 Corinthians Chapter 1 explain this further.
For it is written, I will destroy the wisdom of the wise, and will bring to nothing the understanding of the prudent. Where is the wise? where is the scribe? where is the disputer of this world? hath not God made foolish the wisdom of this world? For after that in the wisdom of God the world by wisdom knew not God, it pleased God by the foolishness of preaching to save them that believe. For the Jews require a sign, and the Greeks seek after wisdom: But we preach Christ crucified, unto the Jews a stumblingblock, and unto the Greeks foolishness; But unto them which are called, both Jews and Greeks, Christ the power of God, and the wisdom of God.