The Jewish people had gotten used to living in a certain way. The Law, given by God on Mt. Sinai, set the expectation. In fact, Jesus begins (or continues) His teaching by quoting part of the Law here in vs. 43 that His listeners would have been aware of, Leviticus 19:18 with its emphasis on loving your neighbor as you love yourself. The breakdown on this verse though is difficult because of Jesus' whole statement, "You have heard it said, love your neighbor, and hate your enemies." This quote is challenging for a couple of reasons. First, the initial phrase is entirely Scriptural coming from Leviticus 19:18. The second though, the "hate your enemies" phrase is not Scriptural at all . . . at least in the same way that "love your neighbor is." Throughout Hebrew Scriptures you are hard pressed to find a verse that orders the Israelites to "hate their enemies." This is why Jesus starts his mini-teaching here with the words "you have heard that it was said . . . " What Jesus is referring to is not Jewish religious teaching or law, but Jewish tradition quite removed from the Law as we see here.
Jesus here is challenging not just official Jewish religious teachings, but how people lived their lives on a day to day basis. Loving your neighbor and hating your enemy is an easy thing. Hating your enemy is lazy. Hating your enemy is the default, right? With the introduction or advent of the Kingdom of God all gets set on its proverbial head. "You've heard it said . . . but I tell you." What a challenge. Abandon the old way and adopt my yolk. Even loving your enemy is easier and less pressure than hating them. This of course says something about hate that perhaps we don't have time for here (perhaps another post or comments by you all?).
How do I respond to " . . . but I tell you." Sometimes it's hard. Sometimes I'm led to being choked up a bit because I am free to love, despite who I might disagree with. In these perilous times this is a difficult things; but " . . . but tell you" is revolutionary. ". . . but I tell you" challenges us and forces us to love where love is absent. Love fills the void. Really, can anything else fill the void?