I've read only a few things by Deitrich Bonhoeffer (mostly Life Together and his Prison Poems) and a little about him, but his faith impresses me deeper every time I come across him.
Life Together would make a fabulous Bible study book, personal devotional (about as close as I get to that is reading a couple pages or chapters in something late at night just before sleeping, when the words can sink into me as I drift off to sleep moments later), or I don't know, just something to give as a gift or read on a plane. So many encouraging words for how to deal as a Christian with other people. Encouraging and challenging and convicting and then encouraging again. The best kinds of teaching words. His style sometimes is a bit stifling, ancient, or preachy; but he IS preaching, and it I DID only say sometimes, and you can definitely get past that to find much good.
Often I think of books on this topic (the Christian life) only in the vein of Christian-to-Christian interaction, but while reading this I was struck by how much of it can be used in every area of life, even Christian-to-nonbeliever.
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"The Christian needs another Christian who speaks God's Word to him. He needs him again and again when he becomes uncertain and discouraged... He needs his brother man as a bearer and proclaimer of the divine word of salvation. He needs his brother solely because of Jesus Christ. The Christ in his own heart is weaker than the Christ in the word of his brother; his own heart is uncertain, his brother's is sure." (from the chapter Community)
"For Christians the beginning of the day should not be burdened and oppressed with besetting concerns for the day's work. At the threshold of the new day stands The Lord who made it." (from The Day With Others)
"Word plunges men into the world of things. The Christian steps out of the world of brotherly encounter into the world of impersonal things... an instrument in the hand of God for the purification of Christians from all self-centeredness and self-seeking. The work of the world can be done only where a person forgets himself, where he loses himself in a cause, in reality, the task, the 'it.'" (The Day With Others)
"Each by itself has profound pitfalls and perils. One who wants fellowship without solitude plunges into the void of words and feelings, and one who seeks solitude without fellowship perishes in the abyss of vanity, self-infatuation, and despair. Let him who cannot be alone beware of community. Let him who is not in community beware of being alone." (The Day Alone)
"To make intercession means to grant our brother the same right that we have received, namely, to stand before Christ and share in His mercy." (The Day Alone)
"I can never know beforehand how God's image should appear in others. That image always manifests a completely new and unique form that comes solely from God's free and sovereign creation. To me the sight may seem strange, even ungodly. But God creates every man in the likeness of His Son, the Crucified. After all, even that image certainly looked strange and ungodly to me before I grasped it." (Ministry)