Luke begins with an introduction that sounds a lot like introductions of historians of the period. The historical introduction usually functioned to summarize the case the historian wished to make. For example, Josephus in his preface to Jewish Wars defended the Jews by explaining that only a fringe group, the Zealots, were responsible for the uprising that led to the eventual destruction of Jerusalem in 70 A.D. Luke, in a similar way, writes in order to explain the roots of a movement called "the way" in his second volume, Acts.
But Luke's interest was not merely historical. Indeed, most ancient historians rarely wrote history merely to recount facts. There was a reason behind including what they did in their material. This was usually stated in the introduction. Luke's purpose was not merely historical, but theological. He recounted what he did in order to solidify faith. Notice,
"Inasmuch as many have undertaken to compile an account of the things accomplished among us, just as they were handed down to us by those who from the beginning were eyewitnesses and servants of the word, it seemed fitting for me as well, having investigated everything carefully from the beginning, to write it out for you in consecutive order, most excellent Theophilus; so that you may know the exact truth about the things you have been taught (Luke 1:1-4)."
Luke's purpose was to for his readers to know the exact truth about the things that were taught. But why write another Gospel? Why not simply refer to Mark or Matthew? Why not write an epistle as Paul so often did? And why go to the trouble of writing using the conventions of secular history writing? The answer may lie in the dedication to Theophilus.
We know almost nothing about Theophilus except that he probably paid for the work. Historical introductions were often dedicated to the patron. Theophilus was a Greek name meaning "God lover." Was he a Gentile, Hellenistic Jew, or a God-fearer? There is no way to know for sure. Whatever he was, Luke wanted his writings to have an air of credibility. If the audience were gentiles or Hellenistic Jews, then the writing would be more credible if it were written in the style of other histories.
Luke went to a lot of trouble to be sensitive to his potential readers. He wanted the message to be credible. When you keep in mind that Luke lived in a pluralistic world that was full of various religions you can see why this was necessary. The situation in Luke's day is not very different from our own. We live in a time that is becoming increasingly pluralistic.
How much trouble do we go to in order to be credible? Where does our credibility come from? We are not writing Gospels or other books of the Bible. That has been completed and the Bible has already proven itself to be credible time and again. Our credibility comes from integrity. The same had to be true in the early church otherwise it would not have grown or survived. When our walk matches our talk, our message will be credible to our neighbors, co-workers, friends, and family.