The Inflexible Schoolmaster
The Protestant Reformers were so certain of the importance of this doctrine (of Law and Gospel) that they declared that without it no one would be able to make sense out of Scripture. Martin Luther even declared of the person ignorant of this distinction that "you cannot be altogether sure whether he is a Christian or a Jew or a pagan, for it depends on this distinction." - Hermann Sasse, Here We Stand: Nature and Character of the Lutheran Faith, trans. by Theodore G. Tappert, (New York: Harper & Bros., 1938). p. 114.
Elsewhere Luther wrote, “Whoever knows well this art of distinguishing between the Law and the gospel, him place at the head and call him a doctor of Holy Scripture.”
“The true knowledge of the distinction between the Law and the Gospel is not only a glorious light, affording the correct understanding of the entire Holy Scriptures, but without this knowledge Scripture is and remains a sealed book.” - C.F.W. Walther, The Proper Distinction Between Law and Gospel.
Theodore Beza said that “confusion of law and gospel is one of the principal sources of the corruptions in the church.” Ursinus, primary author of the Heidelberg Catechism, said the same.
The Bible will be an impenetrable mystery as long as we are confused about this.