As children, we are instructed to be able to one day stand on our own two feet. Jesus had rose from the dead. He appeared to the disciples many times for 40 days. Jesus told them to go to Jerusalem for Pentecost. He said there you will recieve the Holy Ghost and the power of God. It was to be able to preach the gospel to the utter most parts of the world. They watched as Jesus ascended into heaven from the mount of Olives. Then made their way to the upper room. It was the day before Pentecost. While in upper room, Peter, under the direction of the Holy Ghost, spoke to the apostles that they needed to replace Judas with one of their own. Justus and Matthias were chosen. Peter casts lots, urim and thummim, to see who it would be. The lot fell to Matthias. They continued in prayer, because it was almost time for the appearance of the Holy Ghost.