An open ending is one that needs to be completed by the audience, i.e., not all things are resolved, or relevant details are left out. Here is how the book of Acts ends:
Acts 28:30-31: He [Paul] lived there two whole years at his own expense and welcomed all who came to him, proclaiming the kingdom of God and teaching about the Lord Jesus Christ with all boldness and without hindrance.What? This is what the entire narrative has been building toward? Seems a little bit anticlimactic. The book of Acts, like the Gospel of Luke, had a section of building tension. In Acts, Paul, like Jesus in Luke, sets his face to Jerusalem, knowing that he is facing persecution there. In Acts 20 Paul, speaking to the Ephesian elders says:
Acts 20:22-23: And now, as a captive to the Spirit, I am on my way to Jerusalem, not knowing what will happen to me there, except that the Holy Spirit testifies to me in every city that imprisonment and persecutions are waiting for me.Like Jesus in Luke, Paul travels to Jerusalem, undergoing several trials, and finally appealing to the Emperor in Rome. The reader of Acts, who has most likely also read Luke, sees the parallels and is expecting similar results. Yet, there is no satisfaction of these expectations. Paul gets to Rome, and then, ugh, anti-climax. Nothing happens. It says he lives there two years teaching about Jesus. What gives.
Christian tradition has Paul being executed in Rome under the reign of Nero in the mid 60s. Luke, writing 20-30 years after this event was certainly aware of Paul's death. Even the audience of Acts was likely aware of Paul's death. So, why not narrate it? Why leave the audience hanging?
One reason is theological. The book of Acts, imho, is not about Paul, or Peter, or Stephen, or Philip. It is not about a person, or even a group of persons, the apostles. It is about God's work continuing work in the church. By leaving this story open, it makes a profound statement about the fact that this story does not end with the death of Paul, or any other apostle for that matter. This story is still going on. There are still those who are "witnesses in Jerusalem, Judea, Samaria, and to the ends of the earth."