After the suicide of Judas Iscariot, the eleven apostles — having returned from the Mount of Olives — met with the early community in Jerusalem to choose a successor. Peter, citing the Psalms, set the requirement: the new apostle must have been with them 'all the time that the Lord Jesus went in and out among us, beginning from the baptism of John until the day when he was taken up' (Acts 1:21–22).
Two were proposed: Joseph called Barsabbas (also surnamed Justus), and Matthias. The community prayed, cast lots, and the lot fell on Matthias. He was numbered with the Twelve and went out at Pentecost with the rest. Acts then says nothing more of him.
Tradition diverges on his later life. Hippolytus says he preached and died in Jerusalem. Eusebius and the Greek tradition say he preached among the cannibals of Aethiopia (a region near the Black Sea, not modern Ethiopia), or in Cappadocia by the Caspian Sea. His relics were brought, according to the tradition of Trier, by St. Helena, mother of Constantine, partly to Padua and partly to Trier — making St. Matthias's Abbey in Trier the only apostolic burial north of the Alps.