Thomas Brooks emphasizes the genuine heart of a true child of God, who acknowledges their failings yet can confidently declare that they do not harbor any beloved or darling sins. The sermon outlines the believer's hatred for known sins, their active struggle against them, and the deep sorrow they feel over their transgressions. Brooks encourages believers to seek deliverance from sin through holy means, highlighting the joy that comes from overcoming sin. Ultimately, the message is one of hope and the transformative power of God's grace in the life of a believer.