Why is the book of Acts considered a transitional book?
Acts is called a transitional book because it begins as an extension of the Jewish program, with Peter and the eleven appealing to Israel to repent and receive the kingdom. After Israel's rejection, seen in Stephen's stoning, the narrative shifts to Saul of Tarsus, introducing a new phase.
Les teaches that the book of Acts is transitional because it opens as an extension of the Jewish program right after the crucifixion and resurrection. Peter and the eleven are still holding sway, appealing to the nation of Israel to repent of killing their Messiah, and God would yet pick up where He left off, send the king, and they could have the kingdom. Then comes the stoning of Stephen, who also appealed to Israel to believe that the one they killed was indeed the Messiah. At his stoning, the cloaks were laid at the feet of Saul of Tarsus, which is the first introduction to the next man in this transitional book. Saul was a Jew, a Pharisee of the Pharisees, who hated Jesus of Nazareth out of religious conviction and thought he was doing God's service by stamping out early Jewish believers that Christ was the Messiah. Les also identifies chapter 9 as one of the turning points in Acts.