Luke is the longest of the four gospels and the longest book in the New Testament; together with Acts of the Apostles it makes up a two-volume work from the same author, called Luke–Acts.[4] The cornerstone of Luke–Acts' theology is "salvation history", the author's understanding that God's purpose is seen in the way he has acted, and will continue to act, in history.[5] It divides the history of first-century Christianity into three stages, with the gospel making up the first two of these – the arrival among men of Jesus the Messiah, from his birth to the beginning of his earthly mission in the meeting with John the Baptist followed by his earthly ministry, Passion, death, and resurrection (concluding the gospel story per se). The gospel's sources are the Gospel of Mark (for the narrative of Christ's earthly life), the sayings collection called the Q source (for his teachings), and a collection of material called the L (for Luke) source, which is found only in this gospel.[6]