If Luke is writing to the then current high priest of the Jews, the stories he tells of Jesus would have to be important to his overall theme, which is to convict Theophilus of the sins of the priesthood, repent and accept the Gospel of the Kingdom, which offers Jesus as Savior through his crucifixion and resurrection.
That said, what would there be in Luke’s account of Jesus’ birth. Certainly there were other stories that could have been included. Matthew offers another, but Luke speaks of angels and shepherds, Bethlehem and the Temple etc. What is he trying to tell Theophilus? Certainly, Jesus’ circumcision and presentation to God would be important ceremonies and the work of priest in the Temple, as would the account of Mary’s purification 40 days after Jesus’ birth. These things point to the Law and the ceremonies performed at the Temple and would interest Theophilus.
Another consideration that would be a part of Luke’s theme and would be a difficult reminder for Theophilus is Luke’s mention of Bethlehem as the birthplace of Jesus. It was prophesied that the Messiah would be born here (Micah 5:2), but there is something a bit more subtle concerning Jesus’ birthplace. Luke mentions it is David’s city. He was born there being the third generation of Ruth, the Moabitess. This is interesting for two reasons. First, Ruth was not a Jew. She did not come from Jewish stock. Therefore, David, the king, was not of pure Jewish stock! His grandfather was only half Jewish. Theophilus was persecuting people who professed circumcision was not necessary to be accepted by the Lord. Certainly, we can assume by Ruth’s conversation with Naomi that she became a worshiper of the God of Israel, but there is no record in Scripture that she was baptized, which was what both men and women were required to undergo in order to become Jewish proselytes by the time of the 1st century. Yet, in the beginning of Jewish history, no such ceremony seems to have been required. Foreigners simply became worshipers of God, period!
Secondly, the Law stated that that a Moabite could not be taken into the community of the Jews ever (Deuteronomy 23:3-6). But, if this is to be taken without exception, how could Ruth have been received into the community of the Jews at Bethlehem, which is clearly understood that she was accepted if one reads the book of Ruth? The interpretation of these laws was the responsibility of the priests, but especially of the high priest who presided over the Sanhedrin at Jerusalem. If a gentile could not be taken into the community of the Jews (believers in the God of Israel), how could Ruth be received into the Jewish community, especially without having to undergo the ceremonial baptism rite that was required in the 1st century? This was considered a part of the circumcision rite. The males had to undergo the actual operation of the cutting of the flesh, but both men and women were required to undergo ceremonial baptism (washing, cleansing). Nevertheless, the Scripture is silent about such a procedure with regard to Ruth. How should Theophilus interpret this in light of his persecuting fellow Jews who professed circumcision was not necessary to be accepted by the Lord?