Isaak Theophil Keller was born 25 February 1842 in Karras (North Caucasus), son of Melchior Keller, a teacher, and Johanna Margaret Hübner. He died in 1918 in Katharinenstadt. He married Anna Hedwig Dell on 26 August 1869 in Dorpat, Estonia. She was born 11 June 1843, the daughter of Andreas Paul Dell, a medical corps captain, and Juliane Elisabeth Dyrsen.
From 1853 to 1859, he attended St. Anne's School in St. Petersburg. From 1859 to 1861, he worked at Wiedemann's sanatorium in St. Petersburg. From 1861 to 1868, he was a student of philosophy and theology in Dorpat, and on 7 July 1869 he was ordained. He served from 1869 to 1873 as pastor in Fresental. From 1873 to 1903, he served as pastor of the Katharinenstadt (North) parish and also served as a teacher. He retired and lived in Moscow for a time.
He is the brother of Pastor Gotthilf Heinrich Keller.
- Schnurr, Joseph. Die Kirchen und das religiöse Leben der Russlanddeutschen – Evangelischer Teil (Stuttgart: AER Verlag Landsmannschaft der Deutschen aus Rußland, 1978): 149.