An excerpt from Oswald Bayer’s Theology the Lutheran Way:

(On Psalm 51)
The words of the psalm compel us to speak of sin and grace. Theology can have no other theme. But even sin and grace can be spoken of philosophically, meaning “metaphysically”, “morally”, or “historically.” The “rule” Luther uses to judge the genuineness of theology is this: “By divine promises and laws, not by human rules, ‘so that you are justified in your words'” (v.4). To speak theologically of sin and grace means to speak of God’s promise (promissio) and of his law (lex), of the accusing and killing law (Gesetz) and the comforting and life giving gospel (Evangelium).