K. A. Heinrich (Karl Adam Heinrich) Kellner

Born in Heiligenstadt on August 26, 1837, and died in Bonn on February 6, 1915, Karl Adam Heinrich Kellner came from a family of educators. He studied philosophy, theology, and philology in Münster and Tübingen, continued at the seminary in Trier, was ordained a priest in 1861, and earned a doctorate in Munich soon after.

Kellner served first in parish work, including posts in Trier and Bitburg, before moving into teaching. He became a professor at the seminary in Hildesheim, where he taught subjects including canon law, patrology, and homiletics. Later, after the disruptions of the Kulturkampf, he was appointed to the chair of Historical Theology at the University of Bonn, where he taught church history and pastoral theology.

His best-known book is Heortology: A History of the Christian Festivals from Their Origin to the Present Day, a study of the Church year and the development of saints' feasts. He also wrote on early Christianity, education, and the historical setting of Jesus and the apostles, showing a lifelong interest in how belief, worship, and history fit together.