The Law-Gospel Hermeneutic: Two Different Words...

From chapter 3, The Source of Theology: Revelation of Dr. Michael Horton's sytematic theology:

Both Testaments include both commands and promises. When we speak of the distinction between law and gospel, therefore, we are referring to different illocutionary stances that run throughout all of the Scriptures— everything in both Testaments that is in the form of either an obligatory command or a saving promise in Christ. “Hence,” wrote Luther, “whoever knows well this art of distinguishing between the law and the gospel, him we place at the head and call him a doctor of Holy Scripture.”

Calvin and his Reformed colleagues and theological heirs underscored this point as well. Wilhelm Niesel observes, “Reformed theology recognizes the contrast between law and gospel, in a way similar to Lutheranism. We read in the Second Helvetic Confession: ‘The gospel is indeed opposed to the law. For the law works wrath and pronounces a curse, whereas the gospel preaches grace and blessing.’” Ursinus, chief author of the Heidelberg Catechism, called it “the chief division of Holy Scripture,” and Beza insisted in his catechism that “ignorance of this distinction is one of the causes of the many abuses in the church” throughout history. The great Elizabethan Puritan William Perkins taught that it was the first principle for preachers to learn in interpreting and applying passages. More recently, Herman Bavinck and Louis Berkhof have observed the significance of this distinction for the whole Christian system of faith and practice. J. Van Bruggen adds more recently, “The [Heidelberg] Catechism, thus, mentions the gospel and deliberately does not speak of ‘the Word of God,’ because the law does not work faith. The law (law and gospel are the two parts of the Word which may be distinguished) judges; it does not call a person to God and does not work trust in him. The gospel does that.”

Horton, Michael S. The Christian Faith: A Systematic Theology for Pilgrims on the Way