Leviticus 19:18 (ESV) … “You shall not take vengeance or bear a grudge against the sons of your own people, but you shall love your neighbor as yourself: I am the Lord.”
With nine New Testament references for Leviticus 19:18, it is the most quoted passage in the New Testament. Why is this so? Perhaps the answer is found in James 2:8, where this Old Testament passage is called the Royal Law. It means that this verse was very popular with Jewish scholars such that it was given a title. Note the many New Testament references:
Matthew 5:43, “Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy.”
Matthew 19:18–19, “He saith unto him, Which? Jesus said, Thou shalt do no murder, Thou shalt not commit adultery, Thou shalt not steal, Thou shalt not bear false witness, Honour thy father and thy mother: and, Thou shalt love thy neighbour as thyself.”
Matthew 22:37–40, “Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. This is the first and great commandment. And the second is like unto it, Thou shalt love thy neighbour as thyself. On these two commandments hang all the law and the prophets.”
Mark 12:29–33, “And Jesus answered him, The first of all the commandments is, Hear, O Israel; The Lord our God is one Lord: And thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and with all thy strength: this is the first commandment. And the second is like, namely this, Thou shalt love thy neighbour as thyself. There is none other commandment greater than these. And the scribe said unto him, Well, Master, thou hast said the truth: for there is one God; and there is none other but he: And to love him with all the heart, and with all the understanding, and with all the soul, and with all the strength, and to love his neighbour as himself, is more than all whole burnt offerings and sacrifices.”
Luke 10:27, “And he answering said, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy strength, and with all thy mind; and thy neighbour as thyself.”
Romans 13:8, “Owe no man any thing, but to love one another: for he that loveth another hath fulfilled the law. For this, Thou shalt not commit adultery, Thou shalt not kill, Thou shalt not steal, Thou shalt not bear false witness, Thou shalt not covet; and if there be any other commandment, it is briefly comprehended in this saying, namely, Thou shalt love thy neighbour as thyself.”
Galatians 5:14, “For all the law is fulfilled in one word, even in this; Thou shalt love thy neighbour as thyself.”
James 2:8, “If ye fulfil the royal law according to the scripture, Thou shalt love thy neighbour as thyself, ye do well:”
Similar to the Royal Law is the Golden Rule of Matthew 7:12.
Matthew 7:12, “Therefore all things whatsoever ye would that men should do to you, do ye even so to them: for this is the law and the prophets.” 
The secret of understanding a law is to ask which commandment it follows, what principle it upholds or what kind of person it protects. The keynote is found at the heart of this chapter: ‘Love your neighbour as yourself’ (19:18). This will become known as the Golden Rule, and Jesus will rate it as the second of the great commandments (Matthew 22:39). The first is ‘Love the Lord your God with all your heart and with all your soul and with all your strength’ (Deuteronomy 6:5)—to which Jesus adds ‘and with all your mind’ (Mark 12:30).
The Israelites come to regard their neighbour as anyone who is a fellow Jew (as opposed to a pagan or foreigner). Jesus will show, in his parable of the Good Samaritan, that our neighbour is anyone we have the chance to help—irrespective of race, religion or convenience (Luke 10:25–37). Leviticus anticipates this attitude with the command: ‘Love the alien as yourself’ (Leviticus 19:34).