Right off the bat, Luke admits that his work is one among several attempts to tell the story of Jesus. That's right—we're looking at a remake.
Luke has eye-witnesses and the accounts of secondary reporters to help him out. These shore up Luke's credentials as a careful researcher, even as they underline that he himself is kind of removed from the events.
1:2 is the closest thing to a bibliography we get in Luke. He's no smarmy plagiarizer. Though he doesn't name drop, we're pretty sure he's talking about the Gospel of Mark, which Luke very likely knew.
Luke's work is an A-paper for sure: thorough investigation, accuracy, and order. He and his fellow writers and historians were all about this stuff.
Theophilus was probably a real person, although the literal Greek meaning of this name as one who "loves God" or is "loved by God" makes people think he's just a stand-in for all potential Christian readers. Keep your eye out for this guy in the first verse of Acts, too.
Theophilus is "most excellent" (1:3), but Luke is no Bill and Ted. The title is appropriate for higher-ups in the social orders of the Greco-Roman world.
Ready to impress your friends? Check out this historical tidbit: in a close parallel, the Jewish historian Josephus, who was one of Luke's contemporaries, addresses himself to "most excellent Epaphroditus" (Against Apion). But don't be fooled—addressing your work like this definitely doesn't mean there's an intended readership of one. Instead, well-placed patrons like Theophilus and Epaphroditus were the Twitters of antiquity. They had the means to ensure a work reached a wider audience.
Luke states his purpose of writing: to communicate "the certainty" (1:4 KJV) or "truth" (NRSV) or even "reliability" of the instruction Theophilus has received.