"The preaching of grace can only be protected by the preaching of repentance."

"The promise of grace is not to be squandered; it needs to be protected from the godless.  There are those who are not worthy of the sanctuary.  The proclamation of grace has its limits.  Grace may not be proclaimed to anyone who does not recognize or distinguish or desire it. . . .  The world upon whom grace is thrust as a bargain will grow tired of it, and it will not only trample upon the Holy, but also will tear apart those who force it upon them.  For its own sake, for the sake of the sinner, and for the sake of the community, the Holy is to be protected from cheap surrender.  The Gospel is protected by the preaching of repentance which calls sin sin and declares the sinner guilty.  The key to loose is protected by the key to bind.  The preaching of grace can only be protected by the preaching of repentance."

     Dietrich Bonhoeffer, Collected works of Dietrich Bonhoeffer, ed. Edwin H. Robertson, trans. Edwin H. Robinson and John Bowden, vol. 2, The way to freedom:  letters, lectures and notes 1935-1939 (New York:  Harper and Row, 1966), 151, citing Mt 7:6.  I was put onto this by Eric Metaxas, Bonhoeffer:  pastor, martyr, prophet, spy (Nashville, TN:  Thomas Nelson, 2010), 292-293.
     Statements like this were of course coupled with the practice of confession (to, for example, Bethge).
     This appears in vol. 14 of Dietrich Bonhoeffer Works (Theological education at Finkenwalde:  1935-1937 (Minneapolis:  Fortress Press, 2013)), as "Lecture and discussion on the power of the keys and church discipline"/"Bonhoeffer's lecture on the power of the keys and church discipline in the New Testament," on pp. 830-838:

'Do not give what is holy to the dogs; and do not throw your pearls before swine, or they will trample them under foot and turn and maul you' (Matt. 7:6).  You are not permitted to squander the assurance of grace; it must be protected from the wicked.  There are those who are not worthy of the sanctuary.  The proclamation of grace has its limits.  To those who neither recognize nor distinguish nor desire grace, grace may not be proclaimed.  Not only does doing so defile the sanctuary itself, not only must those who commit this sin also transgress against the most sacred, such abuse of the sacred inevitably also turns against the church-community itself.  The world to which grace is thrown as so much fodder inevitably tires of grace and tramples not only the sacred itself but also those who force it upon the world.  For the sake of the sacred, for the sake of sinners, and for the sake of the church-community, the sacred should be protected against being given away cheaply.  Such protection of the gospel is found in the sermon calling for repentance, which calls sin sin and pronounces the sinner guilty.  Protection of the key to loose is found in the key to bind.  Only within the protection of the penitential sermon can there be a sermon of grace [(827-828, where there are variants)].