The Samaritans were an outcast people living among the Jews in the area between Nazareth and Jerusalem. About seven centuries before our Lord's time, a large number of Jews were taken into captivity in Assyria, and foreigners were settled on the land in their place. They were not at all well received, even though they adopted Jewish customs and the Jewish religion. The priest and the levite, on the other hand, were both of the priestly tribe of the Jews, and among most respected class.
The lawyer, by the accounts we find in the Gospels of Matthew and Mark, was one of the scribes of the temple, and a Pharisee. His question to our Lord seems to have been an attempt to get Him make a mistake -- a complicated question, calculated to make our Lord give an overly simple answer, with which the Pharisees might later embarrass Him. Matthew and Mark have the lawyer asking the question: "Which is the greatest Commandment in the Law?" and our Lord giving the answer. Perhaps they hoped that He would simply pick one of the Ten Commandments as being the most important, so that they could accuse Him of downplaying or belittling the other nine.
In any event, the lawyer asks this question -- "Who is my neighbor?" -- in an attempt to make our Lord's answer seem trivial. If we can't identify just who our neighbor is, it is meaningless to say that we must love him as much as we love ourselves.
It must have come as quite a shock when our Lord told this story, in which the priest and the levite are obviously not good neighbors to the injured man -- and in which this outcast foreigner from Samaria is the hero. Our Lord was saying, in effect, that they should love the Samaritans as they loved themselves, and hold the priestly tribe in consequently lower esteem. Or, more precisely, we should respect people on the basis of their deeds, rather than on their superficial resemblance to ourselves.
It is also significant that in this parable, our Lord is suggesting that eternal life is not some impossible goal that can only be achieved by a tiny number of ultra-holy people. You will notice that the Samaritan did his good deed while continuing to go about his normal business. From the story, he would seem to be a merchant or a businessman or some sort who traveled the road from Jericho to Jerusalem in the course of his business. He was able to continue his good deed by arranging for the injured man to stay at the inn while he continued his journey. On the other hand, the priest and the levite spent their working lives doing the holy work of the Temple, but some how could not find it in themselves to do anything for the man lying by the road. Sometimes holiness manifests itself more powerfully in good works for the unfortunate than it does in folding one's hands in prayer or chanting the Psalms.
Now one might object that the Samaritan could have done more -- he could have stayed to take care of the man himself, or perhaps he could have gone off to notify the man's family. But, again, our Lord was proposing a practical standard of behavior, knowing full well that most people living in the world cannot just drop everything to render an act of charity. The Samaritan, after all, would have had no money to pay the innkeeper for the injured man's care if he didn't get back on the road and tend to his business. Perhaps it is also that our Lord recognizes human frailty, and knows that there would be even less charity in the world if all charity had to be done with complete devotion.
We might turn that around, and say that world would be a much better place if everyone exercised the same degree of charity as the Samaritan. Even though his efforts may not have been perfect, they were adequate -- and certainly a great relief of the iinjured man's suffering. And, imagine how much easier it would have been for everyone concerned if all three travelers had taken an interest -- if each one had simply done what was reasonable for him to do.
Saint Paul hints at the same thing in his epistle. Christianity is not a religion of "checklists" -- not a religion that is fulfilled by carrying out a large number of ritual prescriptions and thereby automatically becoming holy. To be sure, there are things that we must do, and things that we must not -- but by and large, our salvation will be more by the spirit of the law than by its exact letter. As God loves us, we must love Him and love one another. That love may be shown in a vast number of ways, most of which cannot be written down and checked off.
"What must I do to gain eternal life?" Our Lord's answer is rather simple, isn't it? Not all that difficult a thing to do, considering the reward.