The Universal Principle?
"Do to others as you would have them do to you." (6:31)
Sound familiar? Yeah, that's the golden rule.
We've all heard something like this since our kindergarten days: "Bad boy! How would you like it if she took your crayon!" It's pretty convincing, even for six-year-olds, and really is a good rule of thumb to live by.
While their influence cannot be underestimated, Luke and Matthew (he says it in Matthew 7:12) don't get all the credit for the major cultural importance of this rule. A similar ethical principle appears in several of the world's religions or philosophies, including versions of Stoicism, the Bahá'í Faith, Buddhism, Confucianism, Hinduism, Humanism, Islam, Jainism, Judaism, Wicca, and Zoroastrianism. So yeah, it's kind of all over the place.
In Luke, the saying occurs as one tiny part of a lengthy sermon sometimes called "The Sermon on the Plain" (6:20-49). Before and after the saying, Jesus says that people should love even the people that don't love them back (6:27-30, 32-35).
Yeah. This was as counter-cultural in antiquity as it is in our modern capitalist economies. Reciprocity was, after all, the rule, and relationships flourished on the principle of mutually beneficial exchange. But Jesus argues that it's not good enough to love those who love you back, to do good to those who do good to you, and to loan money to those who can pay you back later. Everybody does that—even criminals. The real challenge is to do all of that with no hope of return until your heavenly payday comes.
Luke is all about ethical standards of compassion, love, and radical charity toward all, and The Golden Rule is no exception.