Christianity in one word: grace. Famously introduced by Paul and even adopted into one of the five solus (sola fide, sola scriptura, sola gratia, solus Christus, and soli Deo gloria): the slogan of Protestant. In Paul's letter to the Ephesians, he wrote:
"For it is by grace you have been saved, through faith--and this not from yourselves, it is the gift of God--" (Ephesians 2:8, NIV)
The interesting question: is it true that Paul is the first to introduce the concept of grace?
It is noteworthy to take into account that some studies indicates Luke, the writer of the gospel, actually was a constant companion to Paul.
The story of Paul is originated in the book Act of Apostles (Acts) which is strongly indicated to be written by Luke as he addressed the book to the same person as his previous book (Gospel of Luke):
"... it seemed good also to me to write an orderly account for you, most excellent Theophilus" (Luke 1:3, NIV)
"In my former book, Theophilus, I wrote about all that Jesus began to do and to teach..." (Acts 1:1, NIV).
And Paul also mention Luke in his letter to the Colossians:
"Our dear friend Luke, the doctor, sends greetings. So does Demas. " (Col 4:14, NIV)
So is it possible that Paul's understanding about Jesus was under influence from Luke? It is highly possible.
Though Luke is suggested to be the one who wrote last (from all synoptic gospels) and took some material from Gospel of Mark and Matthew, it is how he wrote his Gospel that make his Gospel unique compared to the previous two.
Luke tells the story that emphasizing heavily about Jesus as companion to the sinners. As someone who defend them, forgave their sins, and ultimately gave them their salvation. Something that the two previous Gospels (Matthew and Mark) even do not mention!
Let's take a look at a few examples:
Jesus anointed by a sinful woman (Luke 7:36-50)
(compare to Mat 26:6-13, Mark 14:1-11, John 12:1-11)
The others three Gospels mention about a woman that was going to anoint Jesus with perfume stored in an alabaster jar. But a disciple protested that the perfume should be sold instead and the money should be given to the poor.
But only Luke mentions that the woman was a sinner. She stood behind Jesus, at his feet weeping, and wet his feet with her tears. And later on she wiped them with her hair.
The Pharisee saw this and said "If this man were a prophet, he would know who is touching him: a sinner".
And at the end of the story Jesus said to her "Your sins are forgiven."
The Parable of the Lost Son (Luke 15:11-31)
This parable, found exclusively in Luke, tells story about a young son that asked for his share from his father. After he got his share, he set off for distant country and squandered all his wealth for wild living.
Eventually all his wealth was gone, and with nothing to eat, he wanted to fill his stomach with pods that the pigs were eating, but no one gave him anything. Then he intended to go home to his father, but because he had sinned against his father, he only intended to become one of his father hired man.
But as his father saw him, he ran to his son, threw his arms around him and kissed him. As the son spoke his thought to be his father hired man, the father said to his servants to bring the best robe and put it on him. And also to kill the fattened calf and prepare a feast to celebrate.
The Parable of the Pharisee and the Tax Collector (Luke 18:9-14)
This parable also appears exclusively in Luke. It should be noted that at Jesus' time, the Jews were under the occupancy of Roman. Jews as strong monotheistic followers were obliged to pay tribute to Roman, a polytheistic nation. This brought bad notion to the tax collector occupation (as a sinful occupation), moreover because they were said to always collect more than what they supposed to.
Two man went up to the temple and pray, one a Pharisee and the other a tax collector.
The Pharisee prayed "God, I thank you that I am not like the other men -- robbers, evildoers, adulterers or even like this tax collector."
The tax collector stood far away, beat his breast and said "God have mercy on me, a sinner."
And Jesus told that the tax collector went home justified before God.
Zacchaeus the Tax Collector (Luke 19:1-9)
The third story found exclusively in Luke.
As Jesus entered Jericho, a tax collector named Zacchaeus wanted to see Jesus and since he was a short man, he climbed a tree to see him.
Jesus reached the tree, looked up, and said to Zacchaeus that he must stay at his house today. As the people saw this, they began to mutter "He has gone to be the guest of a sinner."
But Zacchaeus stood up and said "Look here and now I give half my possessions to the poor. And if i have cheated anybody, I will pay back four times the amount."
Jesus said to him "Today salvation has come to this house, because this man too is a son of Abraham."
The Two Other Crucified Men at the Crucifixion(Luke 23:39-43)
(compare to Mat 27:38, Mark 15:32, John 19:32)
This is by far the boldest step in Gospel of Luke.
Matthew and John agree that there were two other men crucified together with Jesus. Mark however added that those crucified with him also heaped insults on him.
But Luke tells different story:
One of the criminals who hung there hurled insults at him: "Aren't you the Christ? Save yourself and us!"
But the other criminal rebuked him. "Don't you fear God," he said, "since you are under the same sentence? We are punished justly, for we are getting what our deeds deserve. But this man has done nothing wrong."
Then he said, "Jesus, remember me when you come into your kingdom."
Jesus answered him, "I tell you the truth, today you will be with me in paradise."
Whether Luke conducted more research, or merely crafted those stories is the unknown fact. However through his writings the foundation of grace in Christianity is put. That the path to salvation is not by weighing sins and good deeds but merely because God's love for mankind including sinners.
It is worthy also to look at the last Gospel: John, which is still debatable, the last of the four gospels.
(Gospel of) John mentions literaly:
"For the law was given through Moses; grace and truth came through Jesus Christ." (John 1:17, NIV).
And John also tells the famous story because of its portrayal in Mel Gibson's movie: The Passion. This story however is debatable since nearly all the biblical scholars agreeed that this story is a later addition to the Gospel. New International Version (NIV) stated "The earliest and most reliable manuscripts and other ancient witnesses do not have John 7:53-8:11.
A story, however inaccurate, is always worthy to take a look (John 8:1-11):
The Pharisees brought a woman caught in adultery and said to Jesus: "Teacher, the law of Moses commanded us to stone such women, now what do you say?"
Jesus bent down and started to scribble the ground with his finger. When they kept asking he said "If any one of you is without sin, let him be the first to throw a stone at her."
After hearing this, one by one go away until only Jesus was left. Jesus asked the woman "Woman, where are they? Has no one condemned you?"
"No one, sir" she said.
"Then neither do I condemn you." Jesus declared. "Go now and leave your life of sin."
