The quote is from Luther’s preface to his translation of the New Testament in 1522. On Luther’s distinction between law and gospel, see Roland H. Bainton, “The Bible in the Reformation,” in S.L. Greenslade, ed., The Cambridge History of the Bible, vol. 3, The West from the Reformation to the Present Day (Cambridge: Cambridge Univ. Press, 1963), pp. 19–21.