Always a theologian at heart, he returned to theology in 1864. His Giessen lectures were published under the titles, Ursprung der alttestamentlichen Religion (1865) and Die Propheten des alten Bundes nach ihrer politischen Wirksamkeit (1868). In 1869 appeared his commentary on Hiob, () which stamped him as one of the foremost Old Testament exegetes.

His renown as a theologian was mainly founded on the series of commentaries, based on those of August Wilhelm Knobel's