Luke tells the story of Jesus as a piece of history. His gospel is more like a biography than the other gospels. . . . He is concerned to show the continuity between the story of Jesus and God’s past dealings with his people in OT times and also between the story of Jesus and the rise of the early church. The story of Jesus is part of the ongoing history of God’s activity in the world, but it is the most important part.
By doing this, Luke has shown that the earthly life of Jesus is an essential part of the gospel.