In the 1580s, Loos published a number of works: a prayer book, polemical theological writings against Protestantism, a political work (by subscription) about the Netherlands rebellion, a survey of German Catholic authors, and a pocket Latin grammar book.
In 1585, he moved to Trier, where he observed the witch trials taking place there. Loos first wrote letters to the city authorities, and, failing in that effort, he sought in 1592 to publish a book protesting against the hunts and questioning some of the beliefs of the witch hunters. The attempted publication of De vera et falsa magia (True and False Magic) offended Petrus Binsfeld, the Suffragan Bishop of Trier, and who served as the deputy to Johann VI von Schonenberg, one of the highest ranking officials in the Holy Roman Empire.
Before the book could be printed, the manuscript copy was seized and Loos imprisoned. He was forced to make a public recantation of his errors on his knees before an assembly of church officials, including the Papal Nuncio, in Brussels on March 25, 1593. The manuscript was believed destroyed by the Inquisition and was lost for 300 years.
In the manuscript, Loos argues against the existence of witchcraft and especially against the validity of confessions obtained under torture. (It should be noted that Binsfeld had in 1589 published his own book on witchcraft, in which he supported confessions and denunciations obtained through torture.) In his work, Loos is believed to have been influenced by the arguments of Johann Weyer, a Protestant Dutch physician, who in 1563 put forth a book attacking the persecution of witches while also categorizing kinds of magical demons. After recanting, Loos was under constant watch by religious officials, and was briefly imprisoned several more times, under the accusation that he had relapsed into theological error. This continued persecution was conducted by his nemesis, a priest in the Jesuit order named Martin Del Rio. Loos died February 3, 1595, in Brussels, succumbing to the Plague; Del Rio lamented that Loos had died before Del Rio could have him executed.