Whoever wants to be a Christian will have to put up with his worst enemies calling themselves Christians and with finding false teachers and false Christians in the midst of orthodox teachers and Christians.
The same is true of the human body; it is never totally pure and without blemish. It is not in the nature of our body to be flesh, blood, and bone in all purity. The body harbors certain impurities which it does not retain but expels. The mouth contains saliva; the stomach is full of waste matter; eyes, ears, nose have their discharges, and so on. But it is not at all proper to look at a little child and say, This is not a human being, but a snot-nose. Were a child's mother to hear this, she would retaliate, You scoundrel, what kind of a fool are you? Can't you see beyond the snotty mess? Can't you see the child has a sound body, a fine neck, beautiful eyes, and all the members of a natural sound human being?
So, just as it is true that the human body cannot be totally free of impurities, so also with Christendom, which is a spiritual body, it can never be without corruption and impurity on earth. Were we to eliminate matter, sweat, saliva, and impurities from our natural body, it would become weak. Better for the body to get rid of such impurities itself in normal cleansing manner than for its flesh and blood to become totally corrupt by retaining it. If the Christian church here on earth, therefore, were to be completely pure and without tares, without fanatics, sectarians, and non-Christians, that would not be a good omen. In fact it would be a sure indication that it is not a true spiritual body, that is, not the true church, just as the body cannot be a true natural body if it is without corruption; that the church is mere filth, just as the body putrefies when it no longer expels waste.
Martin Luther, House Postils, (Baker, 1996) 1:268, emphasis added; re: Parable of Wheat & Weeds, Matt. 13:24-30