The first act of the apostles after the Ascension of Jesus was to find a replacement for Judas. With all the questions, doubts, and dangers facing them, they chose to focus their attention on finding a twelfth apostle. Why was this important? 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.
But Jesus had chosen the original twelve. How could they know whom he would choose?
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 would 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 men fit this description -- Matthias and Joseph called Barsabbas. They knew that both these men had been with them and with Jesus through his whole ministry. But which one had the heart to become a witness to his resurrection. 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.