According to legend, he was a follower of Saint Eucharius, the first bishop of Trier. Eucharius was sent to Gaul by Saint Peter as bishop, together with the deaconValerius and the subdeacon Maternus, to preach the Gospel. They came to the Rhine and to Ellelum (Ehl) in Alsace, where Maternus died. His two companions hastened back to St. Peter and begged him to restore the dead man to life. St. Peter gave his pastoral staff to Eucharius, and, upon being touched with it, Maternus, who had been in his grave for forty days, returned to life. The Gentiles were then converted in large numbers. After founding many churches the three companions went to Trier where the work of evangelization progressed so rapidly that Eucharius chose that city for his episcopal residence. Among other miracles related in the legend he raised a dead person to life. An angel announced to him his approaching death and pointed out Valerius as his successor. Eucharius died on December 8, having been bishop for twenty-five years, and was interred in the church of St. John outside the city.
Maternus assisted Valerius for fifteen years and then succeeded him as bishop of Trier for the next forty years. While assisting Valerius, he had already founded the dioceses of Cologne and Tongeren. He also founded a church on the site of a Roman temple which later became Cologne Cathedral. The staff of Saint Peter, with which he had been raised to life, was preserved in Cologne till the end of the tenth century when the upper half was presented to Trier, and was afterwards taken to Prague by Emperor Charles IV.