He protested against the ecclesiastical system, he preached in favour of reform in the Church and advocated a return to the poverty recommended by the Scriptures. Indeed, the Scriptures were the only rule and every man had the right to study them. In Questio de indulgentis (1412) he denounced the indulgences.
He admired Wyclif’s writings and defended him when he was condemned as a heretic. He was excommunicated. An interdict was pronounced over Prague and he had to leave it and go to southern Bohemia, where he preached and wrote theological treatises, notably the Tractatus de ecclesia (1413), known as «The Church».