It is one of the fundamental commandments of the Torah (the Pentateuch or Five Books of Moses). It is exemplary of Jewish morality, and it very early characterized the Christian faith as well. For 2,000 years, however, it has been misinterpreted.
The commandment appears in Leviticus. Moses is speaking, reciting a long series of laws. You shall not hate your brother, he tells them (Leviticus 19:17), nor take vengeance nor bear a grudge against the sons of your own people (Leviticus 19:18):
“But you shall love your neighbor as yourself: I am the Lord” (Leviticus 19:18).
In the Gospels, Jesus is asked which is the first commandment (Mark), the greatest commandment (Matthew), the law that will bring eternal life (Luke). The question varies slightly in the three Synoptic Gospels, but Jesus’ answer is the same in all three (Matthew 22:37–40; Mark 12:29–31; Luke 10:27). He quotes two passages from the Hebrew Bible: “You shall love the Lord” (Deuteronomy 6:5) and “You shall love your neighbor as yourself” (Leviticus 19:18).
The venerable Rabbi Akivah (first century C.E.a) declared this “a great principle in the Torah” (Genesis Rabbah 24, 7).