Fidelis was born Mark Rey, in the town of Sigmaringen in Germany in the year 1577. He was born to wealth, power and prestige. He studied Civil Law at Freiburg in Germany and was a practicing lawyer who was known for his zealous advocacy for the poor when he entered the Capuchin Order. He was given the name Fidelis, which means 'faithful', a very fitting name. He led an austere life in vigils and prayers. After his ordination to the priesthood in 1612, he dedicated himself to preaching God's Word. The Congregation for the Propagation of the Faith commissioned him in 1612 to preach sound doctrine in South Germany and Switzerland. He labored for ten years in this area when, in 1622, he was attacked by heretics at Seewis in Switzerland and died a martyr’s death. This same year the Congregation Propaganda Fidei was formally established and Fidelis was proclaimed the patron of the Propagation of the Faith.
He was beatified by Pope Benedict XIII on March 12, 1729, and canonized by Pope Benedict XIV on June 29, 1745.