He means to say it is both old and new. Leviticus 19:18 reads, "Do not seek revenge or bear a grudge against one of your people, but love your neighbor as yourself. I am the Lord." (emphasis mine) Jesus said that we should love one another as he loved us, which is more than loving our neighbor as ourselves (John 13:34). Loving someone as God loves us goes far beyond loving someone as we love ourselves. In this way, Jesus' teaching is an old teaching with a new twist. It is a command of a greater magnitude. The Jewish hearers of Jesus' teaching would have noticed the change immediately. They had been taught to love only those who did good to them.
But Jesus' teaching specifically went beyond even loving our neighbors when he proclaimed, "Love your enemies and pray for those who persecute you" (Matthew 5:43). So the teaching of Jesus progressed from the old command to "love your neighbor as yourself" to "love your neighbor as I love you" to "love your enemies". Changing from "love your neighbor" to "love your enemy" indeed makes it a new command. Undoubtedly this is one of the hardest teachings of Jesus to accept and try to practice.
For Reflection: As Christians, how are we doing at loving our neighbors? How are we doing at loving our enemies? Perhaps we need to ask ourselves, "Who is my enemy?" Today I hear people in the U.S. speak of "those republicans" or "those democrats" as if they are the enemy. Or we speak of unnamed terrorists or ISIS or Al Quaeda as the enemy. Yet Paul says we wrestle not against flesh and blood (Ephesians 6:12). So who, indeed, is our enemy?
Let us pray. Jesus, I seek to follow your command to love my enemies and pray for, or do good to, those who persecute me. Who have I been treating as my enemy? I ask you to help me see them through your eyes. I need help praying for them and not against them.