Why can't an unconverted person understand the things of the Spirit?
Paul writes that the natural, unconverted man cannot understand the things of the Spirit because they are spiritually discerned; they are foolishness to him, and he receives them not. Understanding comes only when God opens eyes and hearts, and that is left to His timing.
Les points to 1 Corinthians 2:13-14, where Paul contrasts human wisdom with the Holy Spirit's teaching. The natural man—unconverted, unregenerated, unsaved, lost—receiveth not the things of the Spirit of God. They are foolishness unto him, and he cannot understand or know them because they are spiritually discerned or understood. Les therefore urges patience with unbelievers, realizing they cannot comprehend these things until God opens their eyes and hearts, and we leave that to His own good timing.
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.