Leviticus 19 begins with the commandment, "You shall be holy, for I the LORD your God am holy" (19:2). The rest of the chapter, then, instructs the Israelites how to "be holy." The list of laws includes many having to do with relationships, from honoring parents (19:3) to loving the aliens who live among the Israelites (19:33-34). To "be holy" has to do with treating other people with justice and mercy, caring for the poor (19:9-10), being honest (19:11-13, 35-36), having respect for elders (19:32), and, in general, acting with moral and ethical integrity.
At the heart of these laws is the well-known commandment, "You shall love your neighbor as yourself" (19:18). It is part of a unit of two verses (19:17-18) that instruct the Israelites not to hate one another, not to take revenge or bear a grudge against one another, but to love one another. This verse and fourteen other verses in this chapter end with the refrain of the Holiness Code: "I am the LORD." The point of the chapter seems to be something like this: Because the Lord is holy, and because human beings are made in the image of God, those who are called to emulate God's holiness are to do so by acting with mercy and love toward their fellow human beings.
It is interesting to note a very similar commandment at the end of the chapter, in 19:34: "The alien who resides with you shall be to you as the citizen among you; you shall love the alien as yourself, for you were aliens in the land of Egypt: I am the LORD your God." The commandment to "love your neighbor as yourself" is not to be understood, then, as applying only to one's peers or family. One is also commanded to love the "alien," that is, the foreigner or outsider in one's community. The parable of the Good Samaritan--which begins with the quoting of Leviticus 19:18 and the lawyer's question, "Who is my neighbor?"--makes much the same point (Luke 10:25-37).
Leviticus 19:18 is, of course, most well-known because Jesus cites it as the second part of the greatest commandment. Asked by the religious authorities which commandment is the greatest, Jesus responds by quoting Deuteronomy 6:5 and Leviticus 19:18: "'You shall love the Lord your God with all your heart, and with all your soul, and with all your mind.' This is the greatest and first commandment. And a second is like it: 'You shall love your neighbor as yourself.' On these two commandments hang all the law and the prophets" (Matthew 22:37-40; see also Mark 12:28-34). Given its prominence in this Gospel account, Leviticus 19:18 is arguably the most well-known commandment in the book of Leviticus, though it is likely many Christians do not know the origin of the verse.
18You shall not take vengeance or bear a grudge against any of your people, but you shall love your neighbour as yourself: I am the Lord.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org