The name Matthias occurs in the Acts of the Apostles 1:23.26. After Judas fell out, to make up the number 12, Matthias was chosen by lot to become one of the Twelve. Apparently, the testimony of exactly Twelve must have been perceived as essential. He was one of those who accompanied them during all the time that the Lord Jesus went in and out among them (Acts 1:21).
Tradition has him work in Ethiopia. His remains were supposedly brought to Rome and Trier (Germany) by St. Helena, mother of Emperor Constantine.
A feast in his honor is only attested in the second millennium. While the liturgical calendar in Germany celebrates him on February 24, the universal Roman calendar remembers him on May 14.