In reply, Jesus tells a story of a man lying in the gutter who is ignored by two men who should have known better. If the priest and Levite were serious about their religion, they would have obeyed God's law, and crossed the road. It is left to the Samaritan, the one who by rights should have nothing to do with the law of Israel's God, to do what the law requires and help his neighbour.
But who is the neighbour in the end? It's not a Jew who helps a Samaritan here, but the other way round. The Good Samaritan shows us that Jesus calls us to reach out way beyond our boundaries to the people we would rather not touch. But even more than that, he calls us to let them reach in and touch us.
Luke wants us to know that Jesus is the saviour of the world. Sometimes we Christians can act as though that salvation belongs to us, and we can dispense it to whoever we think deserves it most. But Jesus does not belong to us - he will call whoever he wants, and sometimes we may find that the "Samaritans" have a better grasp of the gospel than we do.