Judas Iscariot had betrayed the Messiah, the Son of God, when it finally dawned on him, how Satan had used him and what he had done, he believed he was damned for all time, and he took his own life.
Twelve disciples had been chosen, called by Jesus, just as there were twelve tribes of Israel. The Twelve were unique. They believed there had to be twelve apostles, for they were “to sit on thrones judging the twelve tribes of Israel” (Luke 22:14,30). So after much discussion they decided to replace Judas the betrayer.
The apostles were of the first generation of believers. They had been together throughout the Lord’s ministry, from the baptism of John to the ascension (Acts 1:21-22). According to Acts, there were two others, who had also followed Jesus throughout his ministry: Joseph called Barsabbas, also known as Justus, and Matthias. Matthias was chosen by lot after prayer to become one of The Twelve. The casting of lots ensures that Matthias is indeed the divine choice. This was confirmed a short time later when Matthias received the Holy Spirit with the others at Pentecost.
A tradition known to Eusebius related that Matthias was one of the seventy (Luke 10:1,17) that Jesus sent out on mission trips. This cannot be proved, but is more likely than Clement of Alexandria’s identification of him with Zacchaeus. One German protestant theologian, Hilgenfeld thinks he is the same as Nathanael in the Gospel of John.
It is generally believed that Matthias ministered in Judea and then carried out missions to foreign places. Greek tradition states that he Christianized Cappadocia, a mountainous district now in central Turkey, later journeying to the region about the Caspian Sea, where he was martyred by crucifixion and, according to other legends, chopped apart. His symbol, related to his alleged martyrdom, is either a cross or a halberd. St. Helena, mother of the Roman emperor Constantine the Great, reputedly transported Matthias’ relics from Jerusalem to Rome.