Fidelis was born Mark Rey in Sigmaringen in Prussia, and was the son of the town's burgomaster. Pursuing studies at the University of Freiburg in Bresigau, he eventually taught philosophy, while working towards a degree in law.
In 1604, he was appointed tutor to a small group of noble youths and with them made a six-year tour of Europe. His pupils, who grew to respect and love him, attested to the austerity and holiness of his life.
On his return to Germany, he took a doctorate in law and was soon known for his integrity and for his espousal of the cause of the oppressed. Still, the corruption within the legal profession disgusted him and he decided to enter the Capuchin branch of the Franciscan Order.
He was a preacher and confessor of great repute and from the beginning of his apostolic life fought heresy, especially in the form of Calvinism and Zwinglianism, not only through preaching but also with his pen.
Appointed, with eight others, apostle of the region of Grison with the mission of bringing its people back to the faith, he undertook the project with courage and dedication. From the start the wonderful effect of his zeal infuriated his adversaries. They roused the peasants against him by spreading the rumor that he was an enemy of their national aspirations and the agent of the Austrian Emperor.
Fidelis was warned, but chose to spend several nights in prayer before the Blessed Sacrament at the feet of a crucifix. On April 24 he was back at his pulpit. A gunshot fired from the crowd missed him, but once back on the road, he was attacked by a group of armed men demanding that he renounce his Faith. He refused and was struck down while calling on God to forgive his assailants, as they mangled his body with their weapons.
The conversion of a Zwinglian minister who witnessed the scene was one of the first fruits of his martyrdom. Fidelis was canonized by Pope Benedict XIV.