Thomas Brooks emphasizes that true faith purifies the heart from sin, likening faith to a diligent housekeeper that not only identifies sin but also actively cleanses it. He illustrates that faith engages Christ's strength to combat sin, transforming the struggle into a divine battle where God fights against sin on behalf of the believer. Brooks highlights that reliance on personal resolutions is futile without Christ, and true victory over sin comes through faith in His power and the application of His blood. He concludes that the only remedy for the spots of sin in a Christian's heart is the cleansing power of Christ's blood, applied through faith.