1Cor 2:14, But the natural man does not receive the things of the Spirit of God, for they are foolishness to him; nor can he know them, because they are spiritually discerned.
The natural man is the unregenerate with the spirit of the world. He is under the dominion of the soul like that of the animal. He does whatever he wants without restraints. He may temporary tame himself to do certain things but his natural animal instincts pulls him back. The things of God are foolishness to him, they don’t make sense and in his animal savage state he cannot discern them.
In Mar 5 & Luk 8, a demon possessed man met Jesus and was freed. The disciples came and found him “clothed and in his right mind”. Before meeting Jesus, he was wild, living in and by the grave yards, cutting himself with stones and wailing. No amount of restraining him with ropes or taming could change him. Like this man, the unregenerate man’s spirit must be stirred from its dormant condition, and be born again, before one can comprehend the things of the Spirit. It is the quickening beams of the Spirit of truth and holiness that must free, renew and help his mind to discern their excellency. Nicodemus was unregenerate man who could not understand Jesus’ sayings concerning salvation until Jesus opened his understanding in order to comprehend.