Upon his return he was much attracted by the personality of Johann Michael Sailer, a friend of the family, at that time professor at the University of Landshut in Bavaria, and studied public finance at that institution.

When Sailer was made Bishop of Ratisbon, Diepenbrock followed him there, took up the study of theology, and was ordained priest 27 December 1823.

His knowledge of modern languages and his administrative ability, together with his understanding of the interior life and his ascetical character, paved the way for his elevation to the episcopal See of Breslau, to which he was elected on 15 January 1845.

The pastoral letter which he issued on this occasion was, by order of the king, read in all the Protestant churches of the realm.

His principal publications are: "Spiritual Bouquet, Gathered in Spanish and German Gardens of Poesy" (Sulzbach, 1826); "Life and Writings of Heinrich Suso" (Ratisbon, 1829); "Sermons" (Ratisbon, 1841); "Pastoral Letters" (Munster, 1853); "Personal Letters" (Frankfort, 1860).