German Protestant (Protestant Heritage) Reformer and theologian who converted from Roman Catholicism to Lutheranism but departed from Martin Luther (Luther, Martin)'s views, emphasizing a mystical attitude in place of dogmatic belief.

A fellow student of the Reformer Martin Bucer (Bucer, Martin) at Heidelberg, Franck was named a curate in the diocese of Augsburg soon after 1516. About 1525 he joined the Lutherans at Nürnberg, giving up his curacy to become a preacher for the Reformation. Franck became disappointed by the moral results of the Reformation, however, and moved away from Lutheranism. At Nürnberg he evidently came in contact with the Anabaptist Hans Denck's disciples, but he soon denounced Anabaptism as dogmatic and narrow. Increasingly at odds with Lutheran doctrines, dogmatism in general, and the concept of an institutional church, Franck moved in 1529 to Strasbourg, which was then a centre of the spiritual movement in Protestantism. There he became a friend of the Reformer and mystic Kaspar Schwenckfeld (Schwenckfeld, Kaspar), who furthered Franck's development as a fierce antidogmatician. Franck's major work, Chronica: Zeitbuch und Geschichtsbibel (1531; “Chronica: Time Book and Historical Bible”), is a wide-ranging history of Christianity that seeks to give heresies and heretics their due.

After a short imprisonment for his views, Franck was expelled from Strasbourg by the civil authorities. He traveled throughout Germany and in 1533 moved to Ulm, where he established himself as a printer. Luther regarded Franck as a man who wanted to avoid both belief and commitment, and the Lutherans at Ulm compelled Franck to leave that city in 1539.

Franck combined the humanist's passion for freedom with the mystic's devotion to a religion based on the inner illumination of the spirit. He believed the Bible was full of contradictions in which true and eternal messages could be unveiled only by the spirit, and he considered dogmatic controversy meaningless. He asserted the extremely antidogmatic notion that Christians need know only the doctrines found in the Ten Commandments and the Apostles' Creed. In the end he became a solitary figure who found no realm of truth left but the inner life of the mystics. Franck's unbiased search for God in various cultures and historical traditions and his emphasis on nondogmatic, nonsectarian, noninstitutional forms of religion mark him as one of the most modern thinkers of the 16th century.