St. Fidelis was born Mark Rey and took the name of "Fidelis" when he joined the Capuchin Order at the age of 35 in 1612. He was born at Sigmaringen, a town in modern-day Germany. He studied law and philosophy at Freiburg. St. Fidelis subsequently taught philosophy at the University of Freiburg, ultimately earning a "doctor of laws". During his time as a student he did not drink wine, and wore a hair-shirt. He was known for his modesty, meekness, and chastity. In 1604, he and three friends travelled through Europe, and during his travels he attended Mass very frequently; in every town where he came, he visited the hospitals and churches, passed several hours on his knees in the presence of the Blessed Sacrament, and gave to the poor sometimes the very clothes off his back.
After he returned home, he took up the practice of law, and was known for his great fairness, and his dislike of ruining anyone’s reputation. He didn’t hesitate to offer his legal help to those who couldn’t afford the cost of a lawyer, and his charity earned him the name of "counsellor and advocate for the poor". He became disenchanted with some of the bad practices associated with many lawyers, and he decided to join the Capuchin friars.
When he entered the Franciscan Order of the Capuchins, he was given the religious name of "Fidelis," meaning Faithful. He finished his novitiate and his studies for the priesthood, offering his first Mass on the feast of Saint Francis of Assisi (October 4), in 1612. As soon as St Fidelis finished his course of theology, he was immediately employed in preaching and in hearing confessions. He was named to be Superior of one of the Capuchin Convents, and many people in the area were renewed in their faith, and several Protestant Calvinists were converted. The Congregation of the Doctrine of the Faith commissioned Fidelis specifically for work among the Protestants.
The Protestants were very angry at this attempt to convert them. They made threats against Fidelis' life, and he began to prepare himself for martyrdom. It was on April 24, 1622, that St Fidelis made his confession, said Mass, and then went out to preach. During the sermon, leaders of the Protestants called for his death. One of them discharged his musket at him in the Church, but missed him, and the Catholics begged him to leave the place, but he was ready to lay down his life. As he went out and was on the road, a group of about twenty Calvinists started to harass him, calling him a false prophet. One of them beat him down to the ground by a stroke on the head with his sword. Fidelis rose again on his knees, and stretching forth his arms in the form of a cross, and prayed to God for their pardon. Another sword struck him in the head, and he fell to the ground and lay in a pool of his own blood. His attackers continued to stab him, and they hacked off his left leg, saying it was punishment for him coming to preach to them. He was buried by the Catholics the next day, and many who had participated in St Fidelis' martyrdom, were converted, and received into the Catholic Church.
O God, who didst enkindle blessed Fidelis with seraphic ardour of spirit in the propagation of the true Faith, and didst vouchsafe to adorn him with the palm of martyrdom, and with glorious miracles: we beseech thee; that, by his merits and intercession, thou wouldest so confirm us through thy grace in faith and charity; that in thy service we may be worthy to be found faithful, even unto death; through Jesus Christ thy Son our Lord, who liveth and reigneth with thee, in the unity of the Holy Spirit, ever one God, world without end. Amen.