Luke commences his gospel by assuring Theophilus that he has done his research...that he has a 'perfect understanding of all things from the very first'.
One of the most important of his discoveries is that he has verified the circumstances surrounding the birth of Jesus. This is, for him, a cause of great celebration. He can demonstrate that their 'messiah' is a descendant of King David through a male line...that Jesus has a claim by right of birth to sit on the earthly throne of David.
Luke sets out to prove his claims to 'perfect understanding' by describing the circumstances surrounding the birth of John the Baptist. He establishes his credentials by going into great detail about the priest Zacharias, even going so far as to identify Zacharias as belonging to the priestly 'course of Abia'.
One of Luke's major purposes here is to identify Zacharias' wife Elizabeth as 'one of the daughters of Aaron'. Thus, husband and wife both belong to the tribe of Levi. He is laying the groundwork, painting an intimate and detailed picture of a priestly family, so that when he goes on to describe the events in Nazareth, Mary will appear no stranger to Theophilus: as a 'suggenes' (tribal kin) of Elizabeth, she will fit neatly into the framework he has already set out.
This crucially important factor has been completely overlooked by theologians intent on promulgating the doctrine of virgin birth.
Coming up : Luke's Testimony Part 2