Luke told Theophilus why he was writing about all that Jesus did.

12 1-2My noble friend Theophilus, many people saw the things that Jesus did while he was with us, from the time when he started [MTY] his ministry. They served God by teaching people the message about the Lord Jesus. Many of those who heard what they taught wrote down for us accounts of the things that Jesus did from the time when he began his ministry. 3 I myself have studied these accounts carefully. So I decided that it would be good for me also to write for you (sg) an accurate account of these matters. 4 I want you to know the truth about what you have been taught {what others have taught you}.