IN THE STATE of Oklahoma, in a hilly area two to three hours east of Oklahoma City, lies Robbers Cave State Park. It’s a favorite of local campers who can even stay in a covered wagon if they choose. The park boasts scenic lakes for fishing and boating, and trails for hiking and horseback riding. And, of course, there are caves to explore.

But why the name “Robbers Cave”? Because in the years after the Civil War, the area was known as a hideout for notorious outlaws, including Jesse James and Belle Starr. Indeed, if camping isn’t your thing, you can stay at Belle Starr View Lodge instead.

But Robbers Cave State Park is also infamous for another reason: it was the site of one of the most widely-known psychology experiments of all time. Though the experiment was conducted in the 1950s, it’s been required reading for psychology students ever since.

Here’s the gist of the study. Twenty-two well-adjusted young boys were brought to the park and split into two groups. Neither group knew that the other existed. Over the course of several days, the members of each group bonded with each other through shared activities like swimming and hiking. One group decided to call themselves The Eagles, while the others decided to be The Rattlers.

The experimenter then brought the groups together and stoked competition between them through team activities like baseball and tug-of-war. The winners got prizes, and the losers got nothing. You can see where this is going. Over the next few days, the competition between the groups escalated and became both personal and aggressive — so much so that the groups had to be forcibly separated at times. Not surprisingly, when they were asked, each group described themselves in positive terms and the other group negatively.

The good news is that the animosity between the groups could be reversed. When the experimenters engineered crises that required everyone to work together for the common good, the boys banded together and the conflict between the groups diminished. It was no longer “us versus them,” in-groups and out-groups, but all of us together.

Since that time, it has been demonstrated again and again just how easily and automatically we as human beings form in-groups and out-groups, sometimes for the most superficial of reasons. Indeed, some argue that this is partly a result of how our brains are wired. But in the same way that it was demonstrated in the Robbers Cave experiment, dividing the social world into categories of “us” and “them” too easily slides into us versus them, friends versus enemies. We’re good, they’re bad. We’re right, they’re wrong. And that perception, in turn, can justify hatred and violence, with each group feeling self-righteous in its cause.

Again, it’s possible to break down such barriers and the hostility that often goes with them. But that can be a steep uphill climb when the prejudice and mutual hostility are long-standing. The Eagles and The Rattlers were only together for a matter of days, while some inter-group conflicts have continued for generations. And even the smallest step toward peace can’t happen if we don’t even realize the ways in which we are creating, reinforcing, and taking for granted in-group and out-group distinctions in the first place.

SO WHY AM I telling you this, and what does it have to do with the Sermon on the Mount?

As we’ve seen, in Matthew 5, Jesus first speaks of the kind of kingdom righteousness that goes beyond that of the scribes and Pharisees, and then illustrates this with a series of antitheses: “Here’s something you’ve been taught about righteousness, but now here’s what I’m telling you instead.” The end of the chapter brings the final antithesis, and it may be the most challenging one of all:

You have heard that it was said, “Love your neighbor and hate your enemy.” But I tell you, love your enemies and pray for those who persecute you, that you may be children of your Father in heaven. He causes his sun to rise on the evil and the good, and sends rain on the righteous and the unrighteous. If you love those who love you, what reward will you get? Are not even the tax collectors doing that? And if you greet only your own people, what are you doing more than others? Do not even pagans do that? Be perfect, therefore, as your heavenly Father is perfect. (Matt 5:43-48, NIV)

If I were to ask you to list your “enemies,” you might scratch your head and tell me that you don’t really have any. After all, you’re a nice person, right? You try to get along with everyone. There’s no one holding a grudge against you, and no one is plotting your downfall (at least that you know of).

But Jesus isn’t necessarily talking about “enemies” in the sense of the drama and conflict needed for a novel or movie. And when he says that we are to love our enemies, he isn’t talking about becoming besties with people who hate us. For that matter, neither does he say that we need to stop acting aggressively toward the people we don’t like.

Notice the counterexamples he gives, illustrating what loving your enemy is not. It is not “loving those who love you.” It is not “greeting your own people.” Is there anything wrong with loving those who love you or greeting your own people? Of course not — and you’d probably find yourself alone pretty quickly if you didn’t. But what Jesus is describing is our automatic, taken-for-granted in-group biases.

Again, he’s not saying that there’s anything intrinsically wrong with loving those who are in our in-group. But anybody can do that. Indeed, everybody already does that, even the people we look down upon. No, if we truly want to be children of our heavenly Father, if we want to know the kind of righteousness that surpasses that of the scribes and Pharisees, we will need to learn to see past the social boundaries we draw between in-groups and out-groups, between us and them, and actively love the people we would normally put on the other side of the line.

WE CAN’T JUST love the people we want to love and think that this is all we need to do to embody the love of God. I imagine that many if not most of the people listening to Jesus were shocked to hear him say that we should love our enemies. But by that time, given what he had already said, they shouldn’t have been surprised.

It was a teaching that was radically different from what they had been taught by the Pharisees. But it was not a new teaching. As we’ll see, there was plenty of precedent in the Old Testament for what Jesus was saying, and apparently, it was the scribes and Pharisees who got it twisted.