Luther, writing in the Large Catechism on the First Petition of the Lord's Prayer...
In the second place, God's name is profaned by an openly wicked life and works, when those who are Christians and God's people are adulters, drunkards, misers, enviers and slanderers (1 Cor. 5:11). Here again God's name must come to shame and be profaned because of us. It is a shame and disgrace for a flesh and blood father to have a bad, perverse child that opposes him in words and deeds. Because of that child the father suffers contempt and reproach. In the same way also, it brings dishonor upon God if we are called by His name and have all kinds of goods from Him, yet we teach, speak and live in any other way than as godly and heavenly children. People would say about us that we must not be God's children, but the devil's children.
From "Luther's Large Catechism with Study Questions," page 105, Concordia Publishing House
