Not a lot is known about Saint Matthias, except that he was one of the disciples of Jesus from fairly early on. He was distinguished by his mortification of the flesh.
After Christ's Ascension, His disciples came together in the Upper Room of the Cenacle in Jerusalem, with Mary and the eleven Apostles and disciples, numbering about 120 in all. They were praying, waiting for the arrival of the Holy Spirit promised by Jesus.
St Peter, in one of his first acts as head of the Church, decided that they must pick a replacement for Judas Iscariot, who had hanged himself in grief over his betrayal of Jesus to the Jews.
There were two disciples considered, both of whom had been with the others for some time, and could attest to the Resurrection of Jesus. One was Joseph called Barsabas, and the other was Matthias. The little company prayed to God that He would enlighten them as to whom He had chosen, and then they cast lots.
Matthias was the one chosen to bring the number of apostles back up to the original twelve.
He later preached in Judea, where he was persecuted by both Jews and Gentiles, and died a martyr being stoned to death in the year 63AD.
His body was taken to Rome some 250 years later by Saint Helena, the mother of Constantine, and the Finder of the True Cross.