1) After Jesus' ascension, the disciples returned to Jerusalem and gathered in an upstairs room with about 120 people, including the remaining 11 apostles. 2) They devoted themselves to prayer along with Jesus' mother, brothers and other believers as they awaited the Holy Spirit. 3) Peter addressed the group and referenced scripture about Judas' betrayal and death, saying they must choose a new apostle to replace him. 4) They selected two men to potentially fill the role, Joseph Barsabbas and Matthias, and cast lots to determine God's choice, which fell to Matthias.