The lawyer who asked the question would have been aware of the nit-picking rabbinical debates over which categories of people could lawfully be excluded from being the ‘neighbours’ that they were commanded to love. In the parable of the Good Samaritan, Christ replies to the question with a parable. His sublime reply is part of general public revelation.
The road from Jericho to Jerusalem was the route to the temple. Service in the temple was arranged by rota so priests and their assistants, the Levites, would often make the journey along that road. Those who performed temple duties were strictly bound by rules that they believed were God-given, rules that protected them from ritual defilement. If they broke the rules they would be unfit to perform their temple duties.
One such rule from the book of Leviticus forbade any contact with a dead body. The lawyer questioning our Lord knew this. The Law would expect a Jewish priest to behave just as the parable says he did. According to the Law, it would have been wrong of the priest not to have passed by on the other side if he thought there was a corpse in the road.
The parable brings a Samaritan into the picture, a man from a despised people. A practising Jew would be defiled even by using a Samaritan drinking vessel. But it is the Samaritan who rescues the traveller. The charity of the untouchable reaches out and touches his neighbour, and that neighbour is not defiled but healed and saved.
The real protagonist of the parable is of course Christ Himself. We are like the man who has been left half-dead. We are travelling along through this dangerous world to our final destiny, the heavenly Jerusalem. Spiritually, we have been attacked and mugged by the evil one, and our soul left half dead. By itself, the Old Law is incapable of saving us. Something new and better is needed. It is supplied by the arrival of the Stranger from outside, the divine Samaritan. He binds up our wounds with love and mercy, and places us in His Holy Church, to be cared for until His return on the Last Day.