The Book of Acts is the second book written by Luke and covers the coming of the Holy Spirit, the growth of the church in Jerusalem, the spread of the Gospel beyond Jerusalem, Paul's missionary journeys and Paul going to Rome.
Acts 1 tells initially of Jesus being taken up into heaven. Proof of Jesus' resurrection is emphasized in that he appeared "over a period of 40 days" and told his followers not to leave Jerusalem but "to wait for the gift my Father promised... and you will be baptized with the Holy Spirit... and you will receive power when the Holy Spirit comes on you, and you will be my witnesses... to the ends of the earth... and after he said this he was taken up before their very eyes... and one day he will return in the same way he went". And there were about 120 of them that stayed in Jerusalem in prayer including Jesus' mother and brothers. Then Peter addressed the group about Judas Iscariot's demise and the need to replace him as an apostle. They chose 2 candidates: Barsabbas (also known as Justus) and Matthias, and after prayer they drew lots which chose Matthias as the replacement apostle.