This quote speaks (in part) to Bonhoeffer’s notion of the world “come of age” and a “religionless Christianity” that has only spoken to God where human knowledge is at its limits. Such cannot be the case. He wrestled with the notion of a positive Christology over and against a simply negative Christology in his lectures while at Finkenwald. In this later development of his thought, he seems yet further arguing for the need to positively construct our theology based on what is known (eg, revealed).
This becomes all the more significant in a world that presses the boundaries of our knowledge yet further and seems to find less need of providing any “theological” explanation for existence and experience (a world which Bonhoeffer found himself wrestling with). Theology cannot be a “stop-gap” to fill the holes of our knowledge. Theology must be located as such in the very concrete (objective and yet subjective) person and work of God in Christ Jesus.