The Gospel of Luke is the longest of the four Gospels despite the fact that it has been divided into four chapters less than was Matthew. It was written primarily for the Gentile Christians to portray Jesus as the Son of Man. In Lu 3:23-38, Jesus' genealogy is traced all the way back to Adam. This is quite different than Matthew's genealogy, which only traces Jesus' ancestry back to Abraham, and this reflects Luke's intent to portray Jesus as the Savior of everyone.
The Gospel of Luke only records nine Old Testament prophecies that Jesus fulfilled, while Matthew (the Gospel to the Jews) records twenty-five prophecies; Mark, eleven prophecies; and John, fifteen prophecies. This is more evidence that this Gospel was written with a Gentile audience in mind.
Luke's desire to present Jesus to the Gentiles can also be seen in many of the events that he alone among the Gospel writers reported, such as the widow's son being raised from the dead at Nain (Lu 7:11-17); Jesus' forgiveness of the sinful woman who fell at His feet despite the Jew's objections (Lu 7:36-50); the parable of the good Samaritan (Lu 10:25-37); the parable of the great supper where the guests wouldn't come (Lu 14:15-24); the parables of the lost sheep and the lost coin, which illustrated His great love for the lost (Lu 15:1-10); the story of the prodigal son and his brother (Lu 15:11-32); and the cleansing of the ten lepers (Lu 17:12-19). The material covered from Lu 9:51 through 18:14 is not given in any of the other Gospels and would be of particular interest to Gentiles. Nearly one-third of Luke's events are peculiar to his Gospel.
Luke was not an eyewitness to the life of Jesus, as were Matthew, John, and possibly Mark. However, he stated that he had a perfect understanding of all the events in Jesus' life, and one of his stated purposes in writing this Gospel was to set those events in their proper order. Thus, Luke was the historian of the Gospel writers.
Luke mentioned in the introduction to his Gospel that many had "taken in hand" to write an orderly account of the life and ministry of Jesus. However, he apparently did not think they succeeded, and therefore he, being led by the Holy Spirit, set out to give us the accurate account. With many accounts in circulation, both written and oral, it is easy to see why an authorized version was needed. The early, extra-biblical sources we have access to mention Luke's Gospel more than any of the other Gospels and show that Luke accomplished his purpose.
Luke alone mentioned the angels' appearance to Zacharias and Elizabeth, and with the exception of seven verses in Matthew, all of the events surrounding the birth of Jesus were recorded by Luke. Luke alone also gave us many of the details of Jesus' life before his baptism by John. These are details that those who were not eyewitnesses themselves would have been interested in and that were apparently missing from the oral accounts.