While serving as a minister in Württemberg, he completed his doctorate and became the lead tutor (Repetent) at the Tübinger Stift in 1891.

From 1894 he was active as a research assistant at the Prussian Academy of Sciences at the instigation of Adolf von Harnack.

Karl Holl's theological development is characterized by the outlook of the “Tubingen school” of Ferdinand Christian Baur.

He published numerous studies on Martin Luther, which made a fundamental contribution to scholarship and remain important today.

Holl's works interpret the Lutheran religious and God concept as “Gewissensreligion” (a religion of conscience) and helped spark the “Luther Renaissance.” He returned the doctrine of justification to its place in the center of theology.