He was almost immediately sent out to preach, in keeping with Dominic's vision of mission of the Order, especially since John was already known as an eloquent preacher, equally fluent in Latin, Italian and French, as well as his native German.

John wandered through northern Italy, France and Germany, as far as Austria, preaching the Gospel everywhere he went, traveling entirely on foot, though he was already advanced in age for his era.

In 1224 he was called to Rome by Pope Honorius III, where he was named an Apostolic Penitentiary and assigned to accompany the Cistercian cardinal Conrad of Urach in preaching a crusade to the Holy Land in southern Germany.

John would travel his extensive diocese on foot, leading, or driving ahead of him, a little ass which carried his books and vestments.

John was not to rest in his retirement, as he was almost immediately sent in 1237 as an envoy of Pope Gregory IX to the Bulgarian Prince Asen II.

The following year, the General Chapter of the Order met in Bologna at Pentecost, in the course of which the capitulars chose John as Prior Provincial of Lombardy, a post in which he served from 1238 to 1240.

This was not a position which offered rest, as friars of that Province were actively opposed to the political ambitions of John's old friend, the Emperor Frederick, who was seeking control of the entire Italian peninsula.

Just as in the start of their friendship, the fact that John was able to walk a fine line which never angered either side in the struggle again attested to his capacity for being able to work with all.

The university monopoly of Paris was broken and from 1248 new general houses of study were established in Montpellier, Bologna, Cologne and Oxford.

In obedience to a command of Pope Innocent IV, John accompanied the first Dominican cardinal, Hugh of St-Cher, to Germany subsequent to the death of Emperor Frederick.