John Gill preaches on 'The Plague of a Man's Own Heart,' emphasizing the inherent sinfulness and corruption of the human heart, which he describes as a grievous disease that affects all of humanity. He explains that while some may be unaware of this plague, those who recognize it are encouraged to seek God's mercy and forgiveness through prayer. Gill highlights the importance of acknowledging our sinful nature and the need for divine healing through Christ, the ultimate physician. He calls believers to humbly confess their sins and rely on God's grace for restoration and pardon.