This article, “I believe in the holy Christian Church,” is as much an article of faith as the rest. This is why natural reason cannot recognize it, even if it puts on all its glasses. The devil can cover it over with offenses and divisions, so that you have to take offense at it. God too can conceal it behind faults and shortcomings of all kinds, so that you necessarily become a fool and pass false judgment on it. Christendom will not be known by sight, but by faith. And faith has to do with things not seen, Hebrews 11[:1]. Christendom joins with her Lord in the song, “Blessed is he who takes no offense at me” [Matt. 11:6]. A Christian is even hidden from himself; he does not see his holiness and virtue, but sees in himself nothing but unholiness and vice. And you, stupid know-it-all, would behold Christendom with your blind reason and unclean eyes!
In a word, our holiness is in heaven, where Christ is; and not in the world, before men’s eyes, like goods in the market place. Therefore let there be offenses, divisions, heresies, and faults; let them do what they can! If only the word of the gospel remains pure among us, and we love and cherish it, we shall not doubt that Christ is with us, even when things are at their worst. As we see here in [Revelation], that through and beyond all plagues, beasts, and evil angels Christ is nonetheless with his saints, and wins the final victory.
Luther’s Works Volume 35, pages 410-411