John Johnson preaches about the life of David, a man after God's own heart, focusing on his choosing, anointing, and his victory over evil. He emphasizes the importance of responding to God's grace from the heart, rather than just outward appearances, and the need for obedience to God's call. Through David's example of facing Goliath, he encourages believers to stand firm in God's power and overcome evil in the world, highlighting the significance of finishing strong in faith and giving praise and glory to God.