There are all sorts of historical events we accept as true, despite a lack of hard evidence – because they were such a big deal that they were widely talked about. The Boston Teal Party is a good example. More than 240 year’s removed, we have very little physical evidence that it actually occurred. But because it was such a big deal, it was talked about far and wide. The story was passed down from one generation to the next. It was recorded by historians who thought it important enough to record.
The birth of Jesus is much the same, at least to me. God went out of his way to make this a really big deal. Even though Jesus’ birth was humble by comparison, God the Father didn’t let it go unnoticed. He made a huge deal out of it. So much so that the story of it was spread far and wide. We still tell the story today – thousands of years later. Luke tells it to Theophilus, again to confirm what he has already heard.