There is only one mention of Matthias in the whole of the New Testament and that is in the Acts of the Apostles, whose author is always understood to have been Luke, the author of the gospel bearing his name. According to the Acts, after the Ascension of Jesus to his Father, the apostles returned to Jerusalem from the Mount of Olives. They gathered in a "room" together with the women who had accompanied Jesus, Mary his mother and other relatives. This room was a refuge because, as friends and companions of Jesus, they were afraid that the authorities might come to arrest them. We are told they gathered there frequently as a group to pray. A few days later, about 120 ‘believers’ gathered together and Peter spoke to them. He wanted to address the problem of choosing someone to replace the traitor, Judas, and keep the number of the Apostles at twelve, a number of significance for these were the ‘patriarchs’ of the New Israel.Peter cited two passages from the Psalms: "Let his encampment become desolate, and may no one live in it" (Ps 69:26) and also, "May another take his office" (Ps 109:8).
The main qualification for the candidate to replace Judas was that he would be someone "who accompanied us the whole time the Lord Jesus came and went among us". Two possible candidates were then chosen – Joseph, called Bar-Sabbas, and Matthias. The gathering prayed that God would indicated which of the two was his choice. They then cast lots, leaving the outcome to God, and the choice was Matthias who was now included among the Twelve Apostles. And that, basically, is all we know about Matthias because there is no other mention of him in the gospels or in any other New Testament writing.
Given such sparse information, it is not surprising that many stories and legends grew around Matthias. He has been identified with a number of other people or even given a different name. St Clement of Alexandria says some identified him with Zacchaeus, the tax collector from Jericho (Luke 19:1-10), while other sources identified him either with Nathanael (John 1:45, 21:2) or with Barnabas (the companion of Paul in the Acts). A legend holds he first preached the Gospel in Judaea and later in Ethiopia or Colchis (now Caucusian Georgia) and that he was crucified in Colchis. A marker placed in the ruins of the Roman fortress at Gonio (Apsaros) in the modern Georgian region of Adjara claims that Matthias is buried at that site. Another legend says: "Matthias preached the Gospel to barbarians and meat-eaters in the interior of Ethiopia, where the sea harbor of Hyssus is, at the mouth of the river Phasis. He died at Sebastopolis, and was buried there, near the Temple of the Sun." Still another tradition says that he was stoned in Jerusalem by Jews and then beheaded but, according to Hippolytus of Rome, he died of old age in Jerusalem.
A Gospel of Matthias (now lost) is attributed to Matthias.
It is claimed that the Apostle’s remains are interred in the oldest German town, Trier, at the Abbey of St. Matthias and were brought there through Empress Helena, mother of the Emperor Constantine I (the Great). However, according to Greek sources, the remains of the apostle are buried in the castle of Gonio-Apsaros, Georgia.
While the Latin Rite celebrates his feast on May 14, the Eastern Orthodox Church celebrates it on August 9.