Luther: "Not all are Christians who boast of faith...."

“Not all are Christians who boast of faith. Christ has shed His blood. We are justified by faith alone without works. You say, “I believe this.” The devil, you say! You have learned the words you have heard the same way mockingbirds learn to repeat things. Where are the fruits demonstrating that you truly believe? You remain in sins; you are a usurer [i.e.,one that lends money at an exorbitant rate]  and more. Surely Christ did not die and shed His blood for the sins that you are intent on committing continually, but so that He might destroy the works of the devil [1 John 3:8]. If you were formerly a usurer, say, like Zacchaeus: “I will give half of my goods, and if I have defrauded anyone, I will restore it fourfold.” [Luke 19:8].

"The blood of Christ kills sin; it does not make it alive, which is the work of the devil, who inflames the desire that makes human beings murderers and adulterers. Christ did not die so that you might remain that kind of sinner, but so that sin, having been slain, might be blotted out, and you might henceforth love God and your neighbor. Faith takes away sins and puts them to death, so that you might not live in them but in righteousness. Therefore, show by your works and your fruits that there is faith in you. If not, the blood of Christ does not help.

"If you are a usurer, disobedient, neglectful of your station, then look to see whether you believe. For faith is victorious, triumphant, a conqueror of the world [1 John 5:4]. If you truly believe, you would not commit usury or adultery; you would not be disobedient. Let each one think: 'I have been made a believer; I have been washed in Baptism with the blood of the Son of God, so that my sins might be dead. [I will] not be disobedient and will declare this with my deeds.” Otherwise, give up the boast of being a believer. You know that you are a disobedient son, an adulterer; do not boast of faith and the blood of Christ. You belong to the devil, the way you are going, etc. You are bringing the name of the Lord into shame and yourself to eternal damnation.”

— Martin Luther, Sermon for the First Sunday after Trinity on 1 John 4:16-21, Preached in St. Mary’s Church, Wittenberg, Germany June 7, 1545. Translated by Christopher Boyd Brown. Unpublished translation. Pr 2002; WA 49:80-87. Copyright Concordia Publishing House, 2010.