|A 19th century depiction of Geert Groote's Brethren of the Common Life|
In 1362 he became a teacher in Deventer, where his success and reputation encouraged him to adopt a lavish lifestyle, until a fellow student, Henry de Calcar, made him see the inappropriateness of this. Groote started to change in the early 1370s, inclining toward mysticism and a more humble life. In fact, he gave his worldly goods to the monastic Carthusians and started preaching a life of repentance.
He gathered a small band of followers who called themselves Broeders des gemeenen levens [Dutch: "Brethren of the Common Life"]. The Brethren did not take vows, but merely gave up their possessions to live in a community where they devoted their waking hours to mass, religious reading and preaching, and manual labor. Meals were eaten as a group, with Scripture read aloud. It had all the hallmarks of a monastic life without the need to be clergy. They founded schools; Nicholas of Cusa attended one.
Before he died (20 August 1384), he organized some of the Brethren who wanted a more "regular" life into Augustinians; he died before this plan was complete, but his disciple Florens Radewyns established a monastery at Windesheim that became the center of up to 100 monastic houses called the Windesheim Congregation.
Groote promoted a style of piety now referred to as Devotio Moderna ["Modern Devotion"]. It was a search for inner peace based on self-deprivation and silent meditation on Christ's Passion. One of the students in his school, Thomas à Kempis, was inspired to write The Imitation of Christ, a work that (I believe) has never been out of circulation since the 15th century.