Lime, Water & Oil – an analogy on the effects of Law and Gospel
Accordingly, the Law is good, righteous, and holy; but it does not justify. It shows me what I am, since through it I am provoked and hate righteousness more than before. It causes me to love lust more than before, since it was only out of terror at the threats of the Law that I was held back from an evil deed, though never from evil lust. And, to put the matter before you by way of a comparison, water is good; but when poured over lime it sets the lime on fire. Is it the fault of the water that the lime becomes hot? Indeed, the lime, which was thought to be cold, is convicted by the water of what it has inside. Thus the Law incites lusts and hatreds, and exposes them; but it does not cure them. But if you pour oil over the lime, it does not become hot; then its hidden heat is extinguished. Thus when grace has been poured into our hearts through the Holy Spirit, it extinguishes hatred and lust.
Luther, Martin Luther’s Works: Volume 27: Lectures on Galatians 275-276