Luke is a doctor and accustomed to careful investigation. He writes Luke and Acts to record the ministry of Jesus and his followers. He speaks of using multiple sources who were eyewitnesses to what he records so he is certain of what he is writing. Luke begins with the story of the birth of John the Baptist. At this time people are not accustomed to miracles. All of this is taking them by surprise, and a level of skepticism is understandable. There probably weren’t very many who believed Mary’s story either. For those who were faithfully following God, it must have been exciting to see God doing miracles like these!
Luke reassures us everything he is recording is real, but many don’t believe it. People have said they would believe the Bible if they could see God do a miracle today, but there were those who didn’t believe in the days these things happened. Regardless of how much evidence people see, some do not want to believe what God is showing them.