Finished the Book of Acts tonight. To sum up, it's basically a narrative about the early Church and what the 12 Apostles did to spread the gospel of Jesus. One of the main elements is how Saul the bad Roman became Paul the good Christian. (I can't help but wonder if Tolkien drew his inspiration for Smeagol/Gollum from this book...)
Acts was written by Luke the Evangelist, aka, the author of the gospel of Luke.
A quick summary: the Apostles elect Mattias to replace Judas (who hanged himself for betraying Jesus) and the Holy Spirit descends on the Twelve who suddenly have the ability to "speak in tongues" and talk to large groups of people wherever they go in their own language, thereby more easily spreading the Word of God. By preaching the story of Jesus, as well as performing miracles (like casting out evil spirits, raising the dead, etc.), the Apostles manage to convert thousands to Christianity wherever they go. But then their numbers grow too big, the authorities freak out, and Christians start getting persecuted. Stephen, accused of blasphemy, is stoned to death, thus becoming the first Christian martyr.
After a while, even the Gentiles (non-Jews) start converting to Christianity. The second half of Acts is devoted to Saul/Paul's story of conversion and mission to spread the gospel. He's loving his role as a persecutor of the new Christians, but then one day is struck blind by a light from heaven and a voice from Jesus asking him why he's persecuting Him (Jesus). Saul is told to go and preach about Jesus, and he makes a sudden conversion when he's given his sight back a few days later -- with scales falling from his eyes, he sees as though for the first time -- and he begins his new life as Paul, one of the most influential Christians ever.
One of the main themes of Acts is the universality of Christianity. Anyone and everyone is welcome and encouraged to join the church and become of follower of Christ.