For thou shalt worship no other god: for the Lord, whose name is Jealous, is a jealous god. —Exodus 34:14

Many verses in the Bible, such as the one above, state unequivocally that Yahweh is a jealous and vengeful lawgiver. They warn of how he loathes idols, and graphically describe the dreadful punishments he will inflict on any of his chosen people who go astray and worship false gods. In fact, the prohibition on worshipping false gods is the very first of the Ten Commandments – more important than abstaining from theft or murder, more important than honoring your parents or being faithful to your spouse.

I’ve often wondered how Jews, Christians and Muslims justify the overriding, supreme importance placed on this rule in their respective religious traditions. Sure, if God existed, I could see him mentioning it here or there – but why stress it this much? Why exalt it above all other moral duties, including those about being kind and loving your fellow people? Are we to believe that the infinite, all-powerful ruler of all existence, the creator of an awe-inspiring universe of a hundred billion galaxies with a hundred billion stars each, feels slighted, or worse, threatened if the inhabitants of one insignificant planet don’t give him what he views as his due praise? Such a depiction speaks to me not of omnipotence and infinite wisdom, but of puny ego and petty jealousy. These are the faults of a vain and insecure human, not the character traits of anything we might reasonably call a god.

No, I think there must be a better reason why the Bible depicts God as all-consumingly jealous. And I think I know what that reason is.

To know more about a god, look to its creators. Why are human beings jealous? The answer, as in many things, lies in evolution. If our male ancestors were unconcerned with their mate’s fidelity, they might well end up using their time and energy to raise other men’s children, rather than propagating their own genes. That variant would soon die out of the gene pool. By contrast, the ancestors who jealously guarded their paternity are the ones whose genes were passed on to us. A similar principle holds for our female ancestors: those who demanded that their mate spend his effort on caring and providing for their own children, rather than the children of another woman, would be far more likely to pass on their genes than those who were indifferent. In both cases, in the brute calculus of reproductive success, jealousy wins and free love loses out.

Richard Dawkins is famous for pointing out that memes, the units of cultural transmission, undergo selection and evolve very much like genes. And we can apply that lesson in this case to come to a surprisingly powerful conclusion: human beings and gods are jealous for the same evolutionary reasons.

After all, what would happen to a religion which taught that its followers could freely mix and mingle with the teachings of other faiths? Like the free-loving ancestor, its memes would soon be mutated and diluted into unrecognizable form. It might survive in some form, but nothing like a coherent set of beliefs or teachings could be passed down over time.

But a jealous god, like a jealous human being, has far better odds of perpetuating itself with fidelity. Stern warnings against worshipping other gods or following other teachings keep the line of transmission pure and ensure that the belief is replicated faithfully from one generation to the next. It may well be that inherently syncretistic belief systems, including modern-day religions like Wicca, simply can never gain much power because they are too diverse. The only kind of religion that can act with ideological unity are the religions of the vindictive and jealous gods – and it’s ironic that those believers who fiercely police the purity of their doctrines, supposing themselves to be doing God’s will, are really just victims of a trick of memetic evolution.