It is in the form of history, we said, that this book of the Gospel was written. So we see that, compared to others, Luke puts his care into reporting facts rather than formulating precepts. Even, in the manner of a story, it begins with a story: “There was,” said he, “in the days when Herod reigned in Judea, a priest named Zacharia,” and he continues until bout this episode. This is the same reason why those who want to recognize in the four animal figures revealed by the Apocalypse the emblem of the four books of the Gospel hold that this one is represented under the features of the bull. The bull is the sacerdotal victim (Leviticus 4:3): there is therefore a relationship between the bull and this gospel which, beginning with the priests, ends with the bull charged with the sins of all and immolated for the life of the whole world. He is the sacerdotal bull. He is both the bull and the priest: the priest, because he intercedes for us – for “we have an advocate,” and it is he, “with the Father” (1 John 2:1) – the bull, for his blood has purified us and redeemed us.
St. Ambrose of Milan, Commentary on Luke, Prologue
Locus Communis 