1 Corinthians 2:14
"14 The person without the Spirit does not accept the things that come from the Spirit of God but considers them foolishness, and cannot understand them because they are discerned only through the Spirit."
The reason so many people in the church cannot understand the things of the Lord is because they were never born-again, or they are lazy Christians who hang onto pride without seeking the Holy Spirit to lead them into truth.
James 1:4-6 "…4 Allow perseverance to finish its work, so that you may be mature and complete, not lacking anything. 5 Now if any of you lacks wisdom, he should ask God, who gives generously to all without finding fault, and it will be given to him. 6 But he must ask in faith, without doubting, because he who doubts is like a wave of the sea, blown and tossed by the wind.…"