Here he published his earliest work (Historisch-kritischer Versuch über die Entstehung u. die frühesten Schicksale der schriftlichen Evangelien), a treatise which had considerable influence on subsequent investigations as to the origin of the gospels. In 1819 Gieseler was appointed a professor ordinarius in theology in the newly-founded University of Bonn, where, besides lecturing on church history, he made important contributions to the literature of that subject in Ernst Rosenmüller's Repertorium, KF Stäudlin and HG Tschirner's Archiv, and in various university "programs."
The first part of the first volume of his well-known Church History appeared in 1824. In 1831 he accepted a call to Göttingen as successor to GJ Planck. He lectured on church history, the history of dogma, and dogmatic theology. In 1837 he was appointed a Consistorialrath, and shortly afterwards was created a knight of the Royal Guelphic Order. The fourth and fifth volumes of the Kirchengeschichte, embracing the period subsequent to 1814, were published posthumously in 1855 by ER Redepenning (1810-1883); and they were followed in 1856 by a Dogmengeschichte, sometimes regarded as the sixth volume of the Church History. Less vivid and picturesque in style than Karl Hase, lacking August Neander's deep and sympathetic insight into the more spiritual forces by which church life is pervaded, he excels these and all other contemporaries in the fulness and accuracy of his information. His Lehrbuch der Kirchengeschichte, with its copious references to original authorities, is of great value to the student: "Gieseler wished that each age should speak for itself, since only by this means can the peculiarity of its ideas be fully appreciated " (Otto Pfleiderer, Development of Theology, p. 284).