The Book of Acts is unique in the New Testament. It is not like the gospels because it does not cover the life and ministry of Jesus. It is unlike the Letter of the Apostles because it is a historical account, not simply a theological letter. This begs the question: what is the purpose of the Book of Acts?
Before we can answer this question, we have to know why the Book of Acts was written. The Book of Acts was not written to be read as we read it today. It was made to be read directly following the Gospel of Luke. Luke and Acts were written as two sections of one combined work: Luke-Acts. The first line of the Book of Acts makes this clear when the writer states, “In my former book, Theophilus, I wrote about all that Jesus began to do and to teach until the day He was taken up to heaven, after giving instructions through the Holy Spirit to the apostles He had chosen.”
Reading the Books of Acts before the Gospel of Luke makes about as much sense as reading The Two Towers before The Fellowship of the Ring. You cannot understand the purpose of the second book without reading the first. Unfortunately, our modern Bibles created this problem. The Gospel of Luke and the Book of Acts are separated by the Gospel of John. This artificial division makes it seem as if they were written as two separate works.
However, if you do read the two books together you can see the purpose of the Book of Acts. The Gospel of Luke ends with Christ's ascension and the Apostles leaving to go pray in the temple. Unlike the other Gospels, it does not have any conclusion or summary sentences. John has the clearest example of this in the final sentences, “Jesus did many other things as well. If every one of them were written down, I suppose that even the whole world would not have room for the books that would be written.” The Gospel of John makes it clear with this statement that this is the end of His story. John has told all that he wishes to tell. Luke doesn’t have a statement like this because while Jesus’s life story might have been finished, for Luke His ministry wasn’t.
That is the purpose of the Book of Acts, to show how Jesus’s ministry lived on in the work of the Apostles. By connecting the books, you see this clear relationship. Jesus’s life is finished, so the Apostles continue with His mission. The Book of Acts shows us that even though Christ is no longer with us physically His spirit still is and we are to carry on with His mission.