After the Ascension of Jesus, the Apostle Peter, in a “general assembly of the faithful” declared the need for a 12th apostle, since Judas was no longer one of the twelve. With all the questions, doubts, and dangers facing the apostles, they chose to focus their attention on finding a twelfth apostle. Twelve was a very important number to the Chosen People: twelve was the number of the twelve tribes of Israel. If the new Israel was to come from the disciples of Jesus, a twelfth apostle was needed.
One hundred and twenty people were gathered for prayer and reflection in the upper room, when Peter stood up to propose the way to make the choice.
Peter 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 simple, 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 and of eating his body — teachings that had made others melt away.
Two were considered as most worthy of the dignity–Joseph, called Barsabas, and, on account of his extraordinary piety, surnamed the Just, and Matthias. Matthias was chosen by lot. 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. He was elected not because he was worthy but because he would become worthy
Matthias was one of the disciples about which little was written. However, the Book of Acts records that he had been a consistent follower from Jesus’ baptism until the Ascension. We do know that he was present at the Pentecost.