During subsequent centuries, there was great fear of the Heresy of the Free Spirit, and many individuals and groups were accused of it. In particular, beguine and beghard groups came under suspicion. John of Dirpheim, Bishop of Strasbourg from 1306 to 1328, was a particularly fervent opponent of heresy. Another person accused, by Bishop John's colleague Henry of Virneburg, Bishop of Cologne, was Meister Eckhart, a German Dominican, who lived during the late thirteenth and early fourteenth centuries. In 1326, Eckhart was charged by the Pope for teaching heresy. He rigorously denied and defended against that charge until he disappeared from public life. Eckhart may have been familiar with the work of Marguerite Porete through his proximity to theologians involved in her trial, such as Berengar of Landora and William of Poitiers. More broadly, as a result of his prominence and through the statements of his used in the bull "In agro Dominico" he came to be recognised by the later mystical tradition as the "father" of the Free Spirit. This is seen particularly in the writings of Jan van Ruusbroec and his followers. During the late fourteenth century, western Germany became a particularly important area for pursuing the heresy. An example of one person executed is the wandering preacher Nicholas of Basel, who was executed sometime between 1393 and 1397. Another known case was the execution of Löffler, who admitted adherence to the movement, in Bern. False beliefs about the annihilation of the will were virulently attacked by the late fourteenth century Theologia Deutsch. In the early fifteenth century, Jean Gerson accused Jan van Ruusbroec of misdescribing the nature of union with God in a way that placed him in the company of the 'Free Spirit' heretics. By the early fifteenth century, the Catholic Church in Germany viewed heresy as a serious threat. It became a leading topic for discussion at the Council of Basel in 1431. Johannes Nider, a Dominican reformer who attended the council, became concerned that beliefs of the Free Spirit heresy, and other heresies, were mixed with elements of witchcraft. In his 1434 work, "Formicarius", Nider combined the Free Spirit heresy with witchcraft in his condemnation of false teachings. "Formicarius" also became a model for "Malleus maleficarum", a later work by Heinrich Kramer in 1486.