I was checking another Luther quote and started reading the whole section and found this jewel.
Then what is a pure heart? In what does it consist? The answer can be given quickly, and you do not have to climb up to heaven or run to a monastery for it and establish it with your own ideas. You should be on your guard against any ideas that you call your own, as if they were just so much mud and filth. And you should realize that when a monk in the monastery is sitting in deepest contemplation, excluding the world from his heart altogether, and thinking about the Lord God the way he himself paints and imagines Him, he is actually sitting—if you will pardon the expression—in the dung, not up to his knees but up to his ears. For he is proceeding on his own ideas without the Word of God; and that is sheer deception and delusion, as Scripture testifies everywhere.
What is meant by a “pure heart” is this: one that is watching and pondering what God says and replacing its own ideas with the Word of God. This alone is pure before God, yes, purity itself, which purifies everything that it includes and touches. Therefore, though a common laborer, a shoemaker, or a blacksmith may be dirty and sooty or may smell because he is covered with dirt and pitch, still he may sit at home and think: “My God has made me a man. He has given me my house, wife, and child and has commanded me to love them and to support them with my work.” Note that he is pondering the Word of God in his heart; and though he stinks outwardly, inwardly he is pure incense before God. But if he attains the highest purity so that he also takes hold of the Gospel and believes in Christ—without this, that purity is impossible—then he is pure completely, inwardly in his heart toward God and outwardly toward everything under him on earth.
Luther, M. (1999, c1956). Vol. 21: Luther’s works, vol. 21 : The Sermon on the Mount and the Magnificat (J. J. Pelikan, H. C. Oswald & H. T. Lehmann, Ed.). Luther’s Works (21:33). Saint Louis: Concordia Publishing House.