The sermon centers on the divine purpose of God's law, clarifying that it was not given to justify humanity but to reveal sin and expose human inability to achieve righteousness through obedience. Paul argues that the law, given through a mediator (Moses) and accompanied by angels, serves as a mirror to show humanity's transgressions and helplessness, not as a means of salvation. Far from being contrary to God's promise, the law fulfills a holy role by driving people to the gospel, where faith alone in Christ alone—whose perfect obedience is imputed to believers—brings justification and life. The sermon further distinguishes between the law's threefold use: to reveal sin, to restrain public evil, and to guide the lives of believers as a standard of holy living, all empowered by grace. Ultimately, the law and the promise are harmonious, both serving God's redemptive plan, with the gospel providing both the basis for righteousness and the power to live in obedience.