That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. John 3.6
Those born of the flesh are those who embrace religions, including Christianity, and do not obey the Word of God. Their nature remains the same.
Like an irrational animal, those born of the flesh continue to possess an irrational nature. They are incapable of understanding spiritual things.
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. I Corinthians 2.14
Those born of the Spirit are spiritual. Their human nature is transformed to the nature of God. They are able to listen, understand, and obey the Voice of the Spirit of God.