Religion

It is, as you say, the only rule that is really needed. If everyone followed it at all times, we'd live in a peaceful world.

It's not the "message of Jesus." It's a universal message that needs no "Savior" to espouse.

The reason it's universal is that it makes perfect logical sense. Nothing it says requires any supernatural entity nor any religion. It's just a simple statement of a fundamental principle of society.

Wouldn't it be much easier to follow that basic rule before taking any action? Think of all the time that would save, compared with long lists of commandments or laws about specific things. All of those derive from the simple rule of reciprocity, anyhow.

Religion and civil law all are based on that single, easy-to-understand rule. Sadly, it is a difficult rule to follow for many people. It shouldn't be, though.

Let's teach the Rule of Reciprocity to everyone, starting in childhood. That should be enough. We don't need all the rest, if people would just follow that single rule.