The first act of the apostles after
the Ascension of Jesus was to find a replacement for Judas. This is described in Acts 1 from verse 15. Challenges faced them from all directions and they had to start by finding a twelfth apostle. Twelve was an important number in Biblical terms starting with the number of
the tribes of Israel.
If the new Israel was to come from the disciples of Jesus, a twelfth apostle was needed. Peter refers to the Book of Psalms, where it is written, that the place deserted by Judas should be filled by another leader.
Jesus chose the original twelve so guidance was required to proceed with making the choice. One hundred and twenty people were gathered for prayer and reflection in the upper room, when Peter stood up to propose the right way. He had one criterion, that, like Andrew, James, John and himself, the new apostle be someone who had been a disciple from the very beginning, from his baptism by John until the Ascension. The reason for this was that the new apostle must become a witness to Jesus' resurrection. He must have followed Jesus before anyone knew him, stayed with him when he made enemies, and believed in him when he spoke of the cross.
Two men fitted this description. They were Matthias and Joseph, also called Barsabbas. They knew that both these men had been with them and with Jesus through his whole ministry. The apostles knew that only the Lord could know what was in the heart of each. They cast lots in order to discover God's will and Matthias was chosen. He was the twelfth apostle and the group was whole again as they waited for the coming of the Holy Spirit.
This is the first and the last that we hear of Matthias in the Gospels but an account by Saint Nicephorus of Constantinople, tells us that Saint Matthias first preached in Judaea, following on to Ethiopia.
Clement of Alexandria says that Matthias, like all the other apostles, was not chosen by Jesus for what he already was, but for what Jesus foresaw he would become. Saint Matthias was chosen because Jesus could see that he would become worthy of the position placed upon him.