In 1604, Rey accompanied, as preceptor (teacher-mentor), three young Swabian gentlemen on their travels through the principal parts of Europe.

In every town they came to, he visited the hospitals and churches, passed several hours on his knees in the presence of the Blessed Sacrament, and was generous to the poor, sometimes giving them the very clothes off his back.

He finished his novitiate and studies for the priesthood, saying his first Mass at the Capuchin friary in Freiburg on 4 October 1612 (the feast day of St. Francis of Assisi, founder of the order).

The Calvinists of that territory, being incensed at his success in converting their brethren, loudly threatened Fidelis' life, and he prepared himself for martyrdom.

The effects of his ardent zeal, about which the Bishop of Coire sent a lengthy and full account to the Congregation for the Propagation of the Faith, enraged the Calvinists in that province.

At the end of his sermon, which he had delivered with more than ordinary zeal, he stood silent all of a sudden, with his eyes fixed upon Heaven, in ecstasy.

Fidelis had been persuaded by the remaining Catholics to immediately flee with the Austrian troops out of Seewis, which he did, but then returned alone to Grüsch.

On his way back he was confronted by 20 Calvinist soldiers who demanded unsuccessfully that he renounce the Catholic faith, and when he refused, they subsequently murdered him.

Fidelis rose again on his knees, and stretching forth his arms in the form of a cross, said with a feeble voice "Pardon my enemies, O Lord: blinded by passion they know not what they do.

The soldiers, not content with this, added many stab wounds to his body with their long knives, and hacked-off his left leg, as they said, to punish him for his many journeys into those parts to preach to them.

The Protestant minister who had participated in Fidelis' martyrdom was converted by this circumstance, made a public abjuration of Calvinism and was received into the Catholic Church.