A teacher of the Law enforces his restraints through threats and punishments. A preacher of grace persuades and incites by calling attention to the goodness and mercy of God. The latter does not desire works prompted by an unwilling spirit, or service that is not the expression of a cheerful heart. He desires that a joyous, willing spirit shall incite to the service of God. He who cannot, by the gracious and lovely message of God’s mercy so lavishly bestowed upon us in Christ, be persuaded in a spirit of love and delight to contribute to the honor of God and the benefit of his neighbor, is worthless to Christianity, and all effort is lost on him. How can one whom the fire of heavenly love and grace cannot melt, be rendered cheerfully obedient by laws and threats? Not human mercy is offered us, but divine mercy, and Paul would have us perceive it and be moved thereby.
– Martin Luther, Church Postil on the Epistle for the First Sunday after Epiphany (Romans 12:1-5)