About 1376 Gerhard retired to this monastery and there spent three years in meditation, prayer and study, without, however, becoming a Carthusian. In 1379, having received ordination as a deacon, he became a missionary preacher throughout the diocese of Utrecht. The success which followed his labors not only in the town of Utrecht, but also in Zwolle, Deventer, Kampen, Amsterdam, Haarlem, Gouda, Leiden, Delft, Zutphen and elsewhere, was immense; according to Thomas à Kempis the people left their business and their meals to hear his sermons, so that the churches could not hold the crowds that flocked together wherever he came.
The bishop of Utrecht supported him warmly, and got him to preach against concubinage in the presence of the clergy assembled in synod. The impartiality of his censures, which he directed not only against the prevailing sins of the laity, but also against heresy, simony, avarice, and impurity among the secular and regular clergy, provoked the hostility of the clergy, and accusations of heterodoxy were brought against him. It was in vain that Groote emitted a Publica Protestatio, in which he declared that Jesus was the great subject of his discourses, that in all of them he believed himself to be in harmony with Catholic doctrine, and that he willingly subjected them to the candid judgment of the Roman Church.
The bishop was induced to issue an edict which prohibited from preaching all who were not in priest's orders, and an appeal to Urban VI was without effect. There is a difficulty as to the date of this prohibition; either it was only a few months before Groote's death, or else it must have been removed by the bishop, for Groote seems to have preached in public in the last year of his life.
At some period (perhaps 1381, perhaps earlier) he paid a visit of some days' duration to the famous mystic John Ruysbroeck, prior of the Augustinian canons at Groenendaal near Brussels; at this visit was formed Groote's attraction for the rule and life of the Augustinian canons which was destined to bear such notable fruit, At the close of his life he was asked by some of the clerics who attached themselves to him to form them into a religious order and Groote resolved that they should be canons regular of St Augustine. No time was lost in the effort to carry out the project but Groote died before a foundation could be made.
In 1387 however, a site was secured at Windesheim, some 20 miles north of Deventer, and here was established the monastery that became the cradle of the Windesheim congregation of canons regular embracing in course of time nearly one hundred houses, and leading the way in the series of reforms undertaken during the 15th century by all the religious orders in Germany. To initiation of this movement was the great achievement of Groote's life; he lived to preside over the birth and first days of his other creation, the society of Brethren of the Common Life. He died of the plague at Deventer in 1384, at the age of 44.
An account, based on these sources, is in S. Kettlewell, Thomas à Kempis and the Brothers of Common Life (1882), i. c. 5; and a shorter account in F. R. Cruise, Thomas à Kempis, 1887, pt. ii. A sketch, with an account of Groote's writings, is given by L. Schulze in Herzog-Hauck, Realencyklopädie (ed. 3); he insists on the fact that Groote's theological and ecclesiastical ideas were those commonly current in his day. and that the attempts to make him a reformer before the Reformation are unhistorical.