Written by Luke, the book of Acts shows us the history of how Christianity spread following the resurrection of Christ. The first half of the book follows Peter and the latter part of the book is focused on Paul. Both men were missionaries spreading the Gospel to the neighboring nations. The book of Acts is considered a part 2 to the book of Luke because he wasn't finished with his story and Jesus wasn't finished after He resurrected.
In verse one Luke brings out the good point about our Lord. He not only taught but He also went about doing what He preached as well. By His actions He sealed His teachings with authority. He gives instructions to His apostles before He ascends to Heaven to wait until the Holy Spirit came upon them before they left Jerusalem to go off on their missions. The Holy Spirit would empower them to do the work they were commanded to do.
There isn't much detail as to what Jesus did the forty days after He was raised from the dead until now. He must have stayed in contact with His disciples because how else did they all know to meet on the Mount of Olives that day. After they asked Him questions about this being His time to prepare His kingdom Jesus didn't give a specific answer to them but reassured them that the Holy Spirit was coming. In verse 8 He tells them to preach to the nations even the ends of the earth. I had never really thought about it but the apostles had no idea just how big this world was. If Jesus wouldn't have told them to reach out they may have just stayed locally and the church wouldn't have spread at that time. After Jesus said all He needed to do He was taken up in the clouds to be seated at the right hand of God until the second coming. The disciples who had watched Him go were greeted by two men, probably angels, who told them He was gone but would return again the way He left. Only Luke talks about the ascension out of all the Gospels. He seemed to keep very detailed accounts of Jesus ministry from beginning to end.
The disciples along with over a hundred others return to the upper room after Jesus' ascension. The upper room was the central place to many of the disciples events previously so it makes sense that they would go there. It was here that Peter stands up and voices the fact that they needed to replace Judas, who had previously hung himself, with a new person. Judas was to be replaced not because he died but because he was defected.There still needed to be twelve disciples to parallel with the twelve tribes. Two men were mentioned, Joseph(Justus) and Mattias. Each of these men needed to be followers of Jesus from the time of John the baptist and must have witnessed the resurrection. After the other disciples had prayed about it and voted they announced it would be Mattias who would fulfill the remaining spot.