Luke's 2nd instalment continues to assure his reader of the truth of Christianity and to defend the church as the authentic people of God. This central purpose of Luke-Acts, confirming the gospel, plays out in the key themes in Acts, including the mission of God and his messenger.
If the gospel of Luke is about "all that Jesus began to do and to teach" (Acts 1:1), the Book of Acts is about what Jesus continues to do through his disciples int he power of the Spirit. The role of the disciples is to be Jesus' witness (1: 8), his representative. Acts 1:8 represents both the central theme of the book (the unstoppable progress of the gospel) and its general outline: beginning in Jerusalem and moving outward in concentric circles thourgh Judea, Samaria, and to the end of the earth.