He was a staunch Lutheran and a prolific writer on theological subjects; his chief field of work was practical theology, and his important book on that subject summing up his long experience and teaching appeared at Erlangen (1877 - 1878, 2 vols.). The liturgy of the Lutheran church of Russia has, since 1898, been based on his Liturgische Formulare (1872).
His son was the German theologian Adolf Harnack (1851 - 1930).