Why did Dietrich Bonhoeffer support a closer relationship between Christianity and humanism?

Dietrich Bonhoeffer supported a rapprochement between Christianity and humanism primarily as a response to the rise of modern tyrannies.1

By rejecting a dualistic separation between the church and the world, Bonhoeffer sought to move away from "thinking in terms of two spheres." Instead, he advocated for a unitive ethic where human activities—such as labor, marriage, and government—are viewed as divinely imposed tasks rather than separate from the sacred. This perspective encouraged a recovery of "the natural" within Protestant thought, allowing the church to engage more directly with the world rather than retreating into otherworldliness.1

Ultimately, Bonhoeffer believed that by affirming humanity’s maturity in a "world come of age," the church could move past a preoccupation with personal salvation and instead focus on "this-worldliness." This shift was intended to free Christians to "share in God’s sufferings in the world" by imitating Jesus, whom he described as "the man for others".1