Johann Arndt (1555-1621), German Lutheran theologian, was born at Ballenstedt, in Anhalt, and studied in several universities.

He was at Helmstadt in 1576; at Wittenberg in 1577. At Wittenberg the crypto-Calvinist controversy was then at its height, and he took the side of Melanchthon and the crypto-Calvinists. He continued his studies in Strassburg, under the professor of Hebrew, Johannes Pappus (1549-1610), a zealous Lutheran, the crown of whose life's work was the forcible suppression of Calvinistic preaching and worship in the day, and who had great influence over him.

In Basel, again, he studied theology under Simon Sulzer (1508-1585), a broad-minded divine of Lutherah sympathies, whose aim was to reconcile the churches of the Helvetic and Wittenberg confessions. In 1581 he went back to Ballenstedt, but was soon recalled to active life by his appointment to the pastorate at Badeborn in 1583. After some time his Lutheran tendencies exposed him to arouse anger of the authorities, who were of the Reformed Church.

Consequently, in 1590 he was deposed for refusing to remove some fixtures from his church and discontinue the use of exorcism at baptism. He found an asylum in Quedlinburg (1590), and afterwards was transferred to St Martin's church at Brunswick in 1599. Arndt's fame rests on his writings. These were mainly of a mystical and devotional kind, and were inspired by St Bernard, J Tauler and Thomas Kempis.

His principal in work, Wahres Christentum (1606-1609), which has been translated into most European languages, has served as the foundation of many books of devotion, both Roman Catholic and Protestant. Arndt here dwells upon the mystical union between the believer and Christ, and endeavours, by drawing attention to Christ's or e in His people, to correct the purely forensic side of the reformation theology, which paid almost exclusive attention with the little anonymous book, Deutsche Theologie. He published an edition of it and called attention to its merits a special preface. After Wahres Christentum, his best-known work is Paradiesgartlein aller christlichen Tugenden, which was published in 1612. Several of his sermons are published in R. Nesselmann's Übers. ü. die Entw. d. chr. Predigt (1858).

Arndt has always been held in very high repute by the German Pietists. The founder of Pietism, Philipp Jakob Spener, repeatedly called attention to him and his writings, and even went so far as to compare him with Plato.

A collected edition of his works was published in Leipzig in 1734. A valuable account of Arndt is to be found in Aschmann's Essai sur la vie, etc., de J. Arndt. See further, Herzog-Hauck, Realencyklopädie.