In the Sermon on the Mount, Jesus attempts to change people's attitude toward Moses' Law from external obedience (that is, "I haven't killed anyone today") to internal obedience (that is, "I have forgiven everyone today").
For example, Jesus says,
You have heard it said long ago, "Do not murder". . . But I say that if anyone is angry with his brother, he will be worthy of judgment. And if anyone says to his brother, "Empty-headed," he will be answerable to the Sanhedrin [the Jewish high court]. But if anyone says, "You fool," he will be in danger of the fire of hell.
Note the progression of Jesus' teaching.
- Don't murder.
- Don't even remain angry.
- Furthermore, don't devalue others by considering them fools.
According to Jesus, when Moses said, "Do not murder," he didn't only mean, "Try to make it through the day without killing anyone," but he also meant, "Don't devalue others by thinking yourself superior to them or harboring anger toward them." For Jesus, devaluing others is akin to (and ultimately the source of) murder.
Jesus goes through the same process with other commands, including adultery ("If you lust over another you've already committed adultery in your heart"), oath taking ("Don't swear oaths," but "Let your yes mean yes, and your no mean no"), retaliation ("If someone strikes you on the right cheek, turn to him the other as well"), and hating your enemies ("Love your enemies" and "pray for those who persecute you"). In case you weren't feeling under the pile already, Jesus concludes this part of his sermon by saying,