Life Together:

Their fellowship is founded solely upon Jesus Christ and this “alien righteousness.” All we can say, therefore, is: the community of Christians springs solely from the biblical and Reformation message of the justification of man through grace alone; this alone is the basis of the longing of Christians for one another. (12)

Thus the very hour of disillusionment with my brother becomes incomparably salutary, because it so thoroughly teaches me that neither of us can ever live by our own words and deeds, but only by that one Word and Deed which really binds us together—the forgiveness of sins in Jesus Christ. When the morning mists of dreams vanish, then dawns the bright day of Christian fellowship. (26–27)

Letters and Papers from Prison:

Over the destiny of woman and of man lies the dark shadow of a word of God’s wrath, a burden from God, which they must carry. The woman must bear her children in pain, and in providing for his family the man must reap many thorns and thistles, and labor in the sweat of his brow. This burden should cause both man and wife to call on God, and should remind them of their eternal destiny in his kingdom. Earthly society is only the beginning of the heavenly society, the earthly home an image of the heavenly home, the earthly family a symbol of the fatherhood of God. [...]

In a word, live together in the forgiveness of your sins, for without it no human fellowship, least of all a marriage, can survive. (31)

The Cost of Discipleship:

Thus it begins; the cross is not a terrible end to an otherwise God-fearing and happy life, but it meets us  at the beginning of our communion with Christ.  When Christ calls a man, he bids him come and die. . . .

“Rejoice and be exceeding glad: for great is your reward in heaven.” There shall the poor be seen in the halls of joy.  With his own hand God wipes away the tears from the eyes  of those who had mourned upon the earth.  He feeds the hungry at his Banquet. There stand the  scarred bodies of the martyrs, now glorified and clothed  in the white robes of eternal righteousness instead of the  rags of sin and repentance. The echoes of this joy reach  the little flock below as it stands beneath the cross,  and they hear Jesus saying: “Blessed are ye!” (99, 128)

Dietrich Beonhoeffer, as quoted in John Piper, This Momentary Marriage: A Parable of Permanence (Crossway, 2009).