Who are the true Christians?

"Here you ask, 'Who are the Christians and where does one find them?' Answer: There are not many of them, but they are everywhere, though they are spread thin and live far apart, under good and bad princes. Christendom must continue to the end, as the article of the creed says, 'I believe one holy Christian church.' So it must be possible to find them. Every pastor and preacher ought diligently to exhort his people to repentance and to prayer. They ought to drive men to repentance by showing our great and numberless sins and our ingratitude, by which we have earned God's wrath and disfavor, so that he justly gives us into the hands of the devil and the Turk. And so that this preaching may work the more strongly, they ought to cite examples and sayings from the Scriptures, such as the flood [Gen. 7:1-24], Sodom and Gomorrah [Gen. 19:24-28], and the children of Israel, and show how cruelly and how often God punished the world and its lands and peoples. And they ought to make it plain that it is no wonder, since we sin more grievously than they did, if we are punished worse than they.
This fight must be begun with repentance, and we must reform our lives, or we shall fight in vain; as the prophet Jeremiah says in chapter 18 [:7-8], 'If at anytime I declare concerning a nation or a kingdom that I will pluck up and break down and destroy it, and if that nation concerning which I have spoken turns from its evil, I will repent of the evil that I intended to do it.' And again, 'And if at any time I declare concerning a nation or a kingdom that I will build and plant it, and if it does evil in my sight, not listening to my voice, then  I will repent of the good which I had intended to do it. Now, therefore, say to the men of Judah and to the inhabitants of Jerusalem, Behold  I am shaping evil against you and devising a plan against you. Return, every one of you and amend your ways and your doings [Jer. 18:9-11]. We may apply these words to ourselves, for God is devising evil against us because of our wickedness and is certainly preparing the Turk against us, as he says in Psalm 7[:12-13], 'If a man does not repent, God will whet his sword; he has bent and strung his bow; he has prepared his deadly weapons.'" - Martin Luther, Selected Writings volume 4, On War Against the Turks, Pg. 18 - 19