Cologne is the oldest of the German bishoprics after Trier. Maternus was the first bishop of Cologne whose name has come down to us. It has been documented that in 313 Emperor Constantine the Great summoned him to a synod in Rome, and that he took part in the Synod of Arles in 314. So we know that the Christian community in Cologne was already significant enough to be presided over by a bishop. Maternus was venerated as a saint. He is remembered in many pictorial representations in the Cathedral and other Cologne churches.
From the official website of Cologne Cathedral
According to an ancient legend, Maternus was a follower of Saint Eucharius, the first bishop of Trier. Eucharius was sent to Gaul by Saint Peter as bishop, together with the deacon Valerius 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.
Valerius was bishop for fifteen years and was succeeded by Maternus, who had in the meantime founded the dioceses of Cologne and Tongeren, being bishop altogether for forty years.
The staff of St. Peter, with which he had been raised to life, was preserved at 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.
Detail of the St Stephen window in Cologne Cathedral