In part this is a response to the church of his day, which relied on Martin Luther's doctrine of Two Kingdoms (Zwei Reiche) to keep the secular and the sacred specifically separate, and to ignore much of what was going on in the secular space, which allowed the church to remain silent in the face of the Third Reich and Adolph Hitler. Even worse, the National Socialists enforced their own kind of unity of profane and sacred in taking over the leadership of the German Church, installing a Reich Bishop, and essentially co-opting the spiritual lives of the German People.

Bonhoeffer, who was a part of the leadership of the breakaway Confessing Church, and who was part of the ecumenical movement for peace, not only separated himself from the German Christians, who stayed with the protestant churches as the Nazis took over, but indeed even felt that the Confessing Church was too accomodating to the threat, trying to save itself rather than speak out too loud. Only the students that Bonhoeffer taught at the Finkenwalde Seminary and later through the Collective Pastorates in Pomerania were anywhere close to where Bonhoeffer was, which was one reason that he ended up in a largely secular and military conspiracy to assassinate Hitler.

In "The Concrete Commandment and the Divine Mandates," Bonhoeffer talks about the Church's commandment to proclaim Jesus Christ as Lord, and talking about it's witness to the world. One Concept is that of "Jesus Christ, the crucified Redeemer," and in discussing this concept, Bonhoeffer states, "A life of genuine worldliness is possible only through the proclamation of the crucified Christ. Thus it is not possible in contradiction to the proclamation, also not beside it in some kind of autonomy of the worldly; but precisely "in, with, and under" the proclamation of Christ that a genuinely worldly life is possible and real." (Ethics pg 400-401, DBWE edition) In using this language, Bonhoeffer evokes the sacrament of communion, and he does so in an uniquely Lutheran manner, using Luther's understanding of the real presence. Just in case you missed it the "nots" are parallel with the Reform and Zwingli points of view in the Reformation struggle over this sacrament. (Zwingli associated with the "contradiction," and the Reformation as "beside it in some kind of autonomy."

This rather rachets up Bonhoeffer's concept doesn't it. It calls the unity of the worldly and the sacred sacramental, not in a wishy-washy way, but exactly in the strong words of Martin Luther, for whom the real presence of Christ in the Eucharist was important enough to prevent the unity of Protestantism in the 16th century, and which resulted in a splintered Protestant church instead of a unified Protestant church.

Evidently for Bonhoeffer, the unity of the worldly and the sacred realm under the Lordship of Christ is exactly that important.