From an article in the Catholic Encyclopedia
Pietism, which was a reaction against the cold and dreary formalism of Lutheran orthodoxy, originated with Philip Spener (1635-1705). In sermons and writings he asserted the claims of personal holiness, and in 1670, while dean at Frankfort-on-the-Main, he began to hold little reunions called collegia pietatis (whence the name Pietist), in which devotional passages of the Scriptures were explained and pious conversation carried on by those present.
His follower, August Francke, founded in 1694 the University of Halle, which became a stronghold of Pietism.
The strict Lutherans accused the Pietists of heresy, a charge which was vigorously denied, although in fact the new school differed from the orthodox not only in practice, but also in doctrine.
The first enthusiasm of the Pietists soon degenerated into fanaticism, and they rapidly lost favour. Pietism had exercised a beneficial influence, but it was followed by the Rationalistic movement, a more radical reaction against orthodoxy, which effected within the Lutheran, as in other Protestant communions, many apostasies from Christian belief. The philosophy of the day and the national literature, then ardently cultivated, had gradually undermined the faith of all classes of the people.
The leaders in the Church adjusted themselves to the new conditions, and soon theological chairs and the pulpits were filled by men who rejected not only the dogmatic teaching of the Symbolical Books, but every supernatural element of religion. A notable exception to this growing infidelity was the sect of Herrnhuters or United Brethren, founded in 1722 by Count von Zinzendorf, a follower of the Pietistic school (see BOHEMIAN BRETHREN).
The critical state of their churches caused many Protestants to long for a union between the Lutherans and the Reformed. The royal house of Prussia laboured to accomplish a union, but all plans were frustrated by the opposition of the theologians. . . .