The salt of the earth
Bonhoeffer says this concerning the Scripture on being "the salt of the earth" (Matthew 5:13-16)

"...the salt may lose its savour and cease to be salt at all. it just stops working. Then it is indeed good for nothing but to be thrown away. That is the peculiar quality of salt. Everything else needs to be seasoned with salt, but once the salt itself has lost its savour, it can never be salted again. Everything else can be saved by salt, however bad it has gone -- only salt which loses its savour has no hope of recovery. That is the other side of the picture. That is the judgement which always hangs over the disciple community, whose mission is to save the world, but which, if it ceases to live up to that mission, is itself irretrievably lost. The call of Jesus Christ means either that we are the salt of the earth, or else we are annihilated; either we follow the call or we are crushed beneath it. There is no second chance." -- chapter 7, The Cost of Discipleship.