Stephen was born in the third century A.D., the son of Jovius, a Roman. Little is known about Stephen's life until he became Archdeacon under Pope Lucius I. Tradition holds that, when about to be martyred, Lucius directed that care of the Church be handed on to Stephen.
Stephen ascended to the pontificate on May 12, 254. As the twenty-third Pope, Stephen "explicitly proclaimed the primacy of the diocese of Rome in matters of theology, and the current understanding of Christ's statement to St. Peter: 'You are Peter, and upon this rock I will build My Church.'" Stephen instructed that special clothing, which came to be our modern-day vestments, must be used for celebrating Mass. These vestments were not to be worn outside of the Eucharistic celebration and street clothes were not to be worn while celebrating Mass.
Pope Stephen also clarified that there was no need to re-baptize heretics, as the Carthagnians were doing. He also made clear that baptisms performed by heretics are valid baptisms so long as none of the essential rites of the sacrament were ommited or changed. This is why a person who is baptized into the Lutheran faith, for example, and subsequently converts to Catholicism, does not need to be re-baptized into the Catholic Church. The Catechism of the Catholic Church states: "Baptism constitutes the foundation of communion among all Christians, including those who are not yet in full communion with the Catholic Church: 'For men who believe in Christ and have been properly baptized are put in some, though imperfect, communion with the Catholic Church. Justified by faith in Baptism, [they] are incorporated into Christ; they therefore have a right to be called Christians, and with good reason are accepted as brothers by the children of the Catholic Church.' 'Baptism therefore constitutes the sacramental bond of unity existing among all who through it are reborn.'"
Old documents have stated that Stephen died as a martyr but no evidence has been found to substantiate this theory. We do know that Stephen died on August 2, 257 and was buried in the Papal crypt of Callistus on the Appian Way. His body was later transferred to St. Stephen's monastery on the orders of Pope St. Paul I. Pope St. Stephen I is the patron saint of Fiano, Romano, Italy.