Any reader of the Bible quickly notices that it gives few details about most of its characters or events. The Eyewitness Bible series of stories are true to scripture, while adding illustrative personalities and personal details to the people who lived these events. These stories also provide historical and geographic context not detailed in Luke.
In the Gospel of Luke, Luke said he made a careful investigation of everything before writing. His three primary sources of information were: his own experiences, written sources, and listening to the stories of those who were personally involved.
Luke had to be very careful writing his book because he was writing it to Theophilus in order to convince him of the truth of the life of Jesus. Theophilus was very likely a patron or influential person.
Luke’s target audience was also anybody else who would read his account, which would likely include people who believed in—and probably worshipped—Roman or Greek gods. Luke was trying to convince everyone that all other gods are false, worthless, and harmful, so he was extremely detailed about the births of John and Jesus. Although miraculous, the births of both John and Jesus really did happen, as opposed to the made-up origins and existence of the gods of Rome and Greece.
Luke spends a lot of time on the birth and life of John the Baptist. He might have done that to prove to his Jewish audience that John had priestly lineage, which would have helped qualify him for a special place as a forerunner of Jesus. It is likely that, at the time of the writing of Luke, the Jews still held John the Baptist to be a prophet. Supporting John’s authority would have the benefit of enhancing Jesus’ public reputation.
Luke is an account of the life of Jesus. This episode explains how Luke felt about the birth story of Jesus. Can you imagine how a doctor like Luke would deal with a virgin giving birth? Luke’s account of Mary’s conception of Jesus is so detailed that at least part of the Bible story surely came straight from Mary. As a doctor, Luke would have asked the most obvious question to Mary, “How can we all be assured that you were a virgin?” Apparently, he resolved that question to his satisfaction.