Here are some other quotes from Bonhoeffer's Life Together, chapter 1:
"One is a brother to another only through Jesus Christ. I am a brother to another person through what Jesus Christ did for me and to me; the other person has become a brother to me through what Jesus Christ did for him. This fact that we are brethren only through Jesus Christ is of immeasurable significance. Not only the other person who is earnest and devout, who comes to me seeking brotherhood, must I deal with in fellowship. My brother is rather that other person who has been redeemed by Christ, delivered from sin, and called to faith and eternal life. Not what a man is in himself as a Christian, his spirituality and piety, constitutes the basis of our community. What determines our brotherhood is what that man is by reason of Christ. Our community with one another consists solely in what Christ has done to both of us. This is true not merely at the beginning, as though in the course of time something else were to be added to our community; it remains so for all the future and to all eternity." (p.25.)