He was born at Dorpat in Russia (now Tallinn, Estonia) where his father, Theodosius Harnack, held a professorship of pastoral theology. Adolf studied at the University of Dorpat (1869-1872) and at Leipzig, where he took his degree; and soon afterwards (1874) began lecturing as a Privatdozent. These lectures, which dealt with such special subjects as Gnosticism and the Apocalypse, attracted considerable attention, and in 1876 he was appointed professor extraordinarius. In the same year he began the publication, in conjunction with Oscar Leopold von Gebhardt and T Zahn, of an edition of the works of the Apostolic Fathers, Patrum apostolicorum opera, a smaller edition of which appeared in 1877.
Three years later he was called to Giessen as professor ordinarius of church history. There he collaborated with OL von Gebhardt in Texte und Untersuchungen zur Geschichte der altchristlichen Litteratur (1882 sqq), an irregular periodical, containing only essays in New Testament and patristic fields. In 1881 he published a work on monasticism, Das Mönchtum--seine Ideale und seine Geschichte (5th ed., 1900; English translation, 1901), and became joint-editor with Emil Schurer of the Theologische Literaturzeitung.
In 1885 he published the first volume of his Lehrbuch der Dogmengeschichte (3rd ed. in three volumes, 1894-1898; English translation in seven volumes, 1894-1899). In this work Harnack traced the rise of dogma, by which he understands the authoritative doctrinal system of the 4th century and its development down to the Protestant Reformation. He considered that in its earliest origins Christian faith and Greek philosophy were so closely intermingled that much that is not essential to Christianity found its way into the resultant system. Therefore Protestants are not only free, but bound, to criticize it; for a Protestant, dogma cannot be said to exist. An abridgment of this appeared in 1889 with the title Grundriss der Dogmengeschichte (3rd ed., 1898).
In 1886 Harnack was called to Marburg; and in 1888, in spite of violent opposition from the conservative church authorities, to Berlin. In 1890 he became a member of the Academy of Sciences. At Berlin, somewhat against his will, he was drawn into a controversy on the Apostles' Creed, in which the party antagonisms within the Prussian Church had found expression. Harnack's view is that the creed contains both too much and too little to be a satisfactory test for candidates for ordination; he preferred a briefer symbol which could be rigorously exacted from all (cf. his Das Apostolische Glaubensbekenntnis. Ein geschichtlicher Bericht nebst einer Einleitung und einem Nachwort, 1892).
At Berlin Harnack continued writing. In 1893 he published a history of early Christian literature down to Eusebius of Caesarea, Geschichte der altkirchlichen Literatur bis Eusebius (part 2 of vol. 5., 1897); and in 1900 appeared his popular lectures, Das Wesen des Christentums (5th ed., 1901; English translation, What is Christianity? 1901). One of his later historical works, Die Mission und Ausbreitung des Christentums in den ersten drei Jahrhunderten (1902; English translation in two volumes, 1904-1905), was followed by some important New Testament studies (Beitrage zur Einleitung in das neue Testament, 1906 sqq.; Engl. trans.: Luke the Physician, 1907; The Sayings of Jesus, 1908).
Harnack was one of the most prolific and stimulating of modern critical scholars, and trained up in his "Seminar" a whole generation of teachers, who carried his ideas and methods throughout the whole of Germany and beyond. His distinctive characteristics were his claim for absolute freedom in the study of church history and the New Testament; his distrust of speculative theology, whether orthodox or liberal; his interest in practical Christianity as a religious life and not a system of theology. Some of his addresses on social matters were published under the heading "Essays on the Social Gospel" (1907).