In comments below, Rob and I revisit an old disagreement about whether "enemy" means "someone you hate" (Rob's position) or "someone who hates you" (my position).
The first time this came up, I took it to be a relatively uninteresting matter of semantics; it's easy enough to figure out which meaning is intended when Rob says, "Christians have no enemies," or I say, "Christians have plenty of enemies."
But Rob's latest comment suggests there's more going on here than that.
Maybe what "enemy" meant to the people who heard Jesus preach was "someone with whom you share mutual hatred." If I am your enemy, then you are my enemy.
A <=HATES=> B
Old idea of "enemy"
If that's what "enemy" means, then the relationship "is an enemy of" has an existence distinct from the people in the relationship. It may be than only one person really chooses to hate the other; it may even be that neither person does. "Jews and Samaritans are enemies," may just be the rule you grow up with, a fact you have to accept as much as, "Gold is valuable."
In that case, Jesus' first task would be to get people to realize that there could be a difference between the people you hate and the people who hate you.
A =HATES=> B
New idea of "enemy"
Then "enemy" loses its independence. It can't exist apart from the will of the one hating. I need not hate you just because you hate me; I need not hate you just because you are a Samaritan and I am a Jew.
Only then would the idea of "loving your enemy" make sense.
A =HATES=> B
B =LOVES=> A
The transition to Jesus' idea of "enemy"
If we only hate other people by free choice, then we are free to choose to never hate other people. I suppose this freedom would come as news to the Jews of Jesus' time, who had till then been required (either by religious prescript or practical necessity) to hate Gentiles, at least to the extent of remaining apart from them, in order to preserve their own identity as Israel. Jesus reveals the full identity of Israel, in which there is neither Jew nor Greek, and so the old idea of "enemy" must be swept away.