The Gospel according to St Luke
Luke is one of the three synoptic gospels (Matthew and Mark being the other two) - so called because they all look at the story of Jesus from a historical perspective, whereas John looks at it from a theological perspective.
The synoptic gospels have a lot in common - sometimes they are word for word. It is therefore believed that the three authors used the same sources of information - either written, oral or both. About 95% of Mark is found in Matthew and Luke. However, if that is removed there is still a lot of material common to both that isn't in Mark. Therefore they both must have used another source. This unknown source is known as "Q" (because Quelle is the German word for source). Then, on top of Mark and Q, Matthew and Luke have their own contributions.
Luke was a gentile, and a doctor by profession (Colossians 4:14 - "Luke our dear doctor"). The book was written to a man called Theophilus. Luke addresses him as "most excellent Theophilus" which suggests that he was a high official in the Roman government. That is why it is often said that Luke was written by the same author as the Acts of the Apostles, for both documents are addressed to the same person (see Luke 1:3 and Acts 1:1).
Date of the book
It is dated AD 75 - 80 because:
Themes of Luke
The Gospel for the Gentiles
Luke very seldom quotes the Old Testament. He tends to give Hebrew words their Greek equivalent - e.g. Calvary is called not by its Hebrew name GOLGOTHA but by its Greek name KRANION, and instead of "rabbi" and "scribe" he writes "teacher" and "lawyer". When giving a list of ancestors of Jesus, Luke traces back to Adam whereas Matthew traces back to Abraham.
The Gospel of Prayer
Luke often shows Jesus at prayer - he prayed at his baptism (3:21), before he chose the twelve disciples (6:12), at the Transfiguration (9:29) and upon the cross (23:46). Only Luke tells us that Jesus prayed for Peter in his hour of testing (22:32).
The Gospel of Women
Women hold an important place in Luke's Gospel which is not found in the Jewish writings - e.g. the birth narrative is told from Mary's point of view and we are told about Martha and Mary, and Mary of Magdalene. Luke talks about the women who accompanied Jesus - Mary (who was called Magdalene), Joanna whose husband Chuza was an officer in Herod's court, Susanna, and "many other women who used their own resources to help Jesus and his disciples" (8:1).
The Gospel of Praise
Some of the most beautiful prayers are only found in this Gospel - the Magnificat (1:46-55), the Benedictus (1:68-79) and the Nunc Dimittis (2:29-32).
The Gospel of the Holy Spirit
In Luke as in Acts, the importance of the Holy Spirit is stressed (1:15, 35, 41, 67 and 2:25-27 and 3:22 and 4:1, 18-21, 10-21 and 11:13)
The Gospel for All
Finally, the Gospel gives the message that Jesus is for all men. The kingdom of heaven is not shut to Samaritans (9:51-56). Luke alone tells the parable of the good Samaritan (10:30-37) and the one grateful leper is a Samaritan (17:11-19) - this can be compared with John (4:9) who talks of how the Jews do not mix with the Samaritans.
Luke shows Jesus speaking with approval of gentiles whom the orthodox Jew would have considered unclean (e.g. 4:25-27). He shows Jesus citing the widow of Zarephath and Naaman the Syrian as shining examples, and he praises the Roman centurion for his faith ("I tell you, I have never found faith like this, not even in Israel" - 7:9). Compare with Matthew 10:5 where Jesus tells his disciples not to go to the Samaritans or gentiles, but Luke omits this.
Luke speaks about the poor. Jesus was laid in a manger and poor shepherds were his first visitors. He talks of how the gospel is preached to the poor ("The spirit of the Lord is upon me because he has chosen me to bring good news to the poor" - 4:18).
Jesus is shown as a friend of outcasts and sinners (7:36-50) and the woman who anointed and bathed Jesus' feet (5:30); he eats and drinks with tax collectors and outcasts; and also the famous Prodigal Son story (15:11-32).
All four Gospels quote Isaiah 40:3-5 "prepare the way of the Lord; make straight in the desert a highway for our God", but only Luke continues with "and all flesh shall see the salvation of God" (cf. Matthew 3:3, Mark 1:3, John 1:23 and Luke 3:4-6).
When you read Luke, the overwhelming thread running through it is God's love for all people, and his joy when they return to him (see Luke 15, the lost coin, the lost sheep and the lost son).
One of the commentaries I read quoted Faber, and I think the following lines encapsulate the essence of the Gospel beautifully: