Therefore we correctly confess in the Creed that we believe a holy church. For it is invisible [hidden], dwelling in the Spirit, in an “unapproachable” place (1 Tim. 6:16); therefore its holiness cannot be seen. God conceals and covers it with weaknesses, sins, errors, and various offenses and forms of the cross in such a way that it is not evident to the senses anywhere. Those who are ignorant of this are immediately offended when they see the weaknesses and sins of those who have been baptized, have the Word, and believe; and they conclude that such people do not belong to the church. Meanwile they imagine that the curch consists of the hermits, monks, etc., who honor God only with their lips and who worship Him in vain, because they do not teach the Word of God but the doctrines and commandments of men. (Matt. 15:8-9) Because these men perform superstitious and unnatural works, which reason praises and admires, they are reguarded as saints and as the church. Anyone who thinks this way turns the article of the Creed, “I believe in a holy church,” upside down; he replaces “I believe” with “I see.” Such forms of human righteousness and self-chosen holiness are actually a kind of spiritual sorcery, by which the eyes and minds of men are blinded and led away from the knowledge of true holiness.
–Martin Luther Lectures on Galatians 1535