So many religious facts have a very long half-life for their
relevance. Say you learn about how the four Gospels differ -- that's
still relevant for understanding Christian divisions or Christian
theology today. Reading about the Reformation? The chance of that
still being relevant is much higher than if you were reading about
purely secular divisions in internal German or Swiss politics in those
Jews, Buddhists, Hindus, Sikhs, or Muslims? Facts from many
centuries ago still might matter. And the odds are that people a few
centuries from now still ought to read about the origins of Mormonism.
In few other areas do past facts stand such a high chance of remaining relevant for so long.
As an empirical matter, "rationalists" tend not to read so much about
religion, but that is precisely the unreasonable thing to do.
I heartily agree, and have several points to add.
1. Studying religion teaches us a tremendous amount about human psychology: our eagerness to embrace doctrines that outsiders consider nonsense, our pretentious overconfidence and hyperbole, our willingness to oppress and even murder over objectively trivial issues.
2. Studying religion also teaches us a tremendous amount about human sociology: our propensity to split into doctrinally-obsessed clergy and angry-sloganizing laymen, our desire to socialize and especially intermarry with co-religionists, our eagerness to dehumanize outgroups.
3. Once you learn these lessons, most secular ideologies start to seem like thinly-veiled religions as well. And I'm not just talking about Marxism. Mainstream liberalism and conservatism fit the same psychological and sociological bill. And yes, so does libertarianism, despite its overrepresentation of bona fide rationalists and individualists. It may be a cliche, but politics is the religion of modernity.
4. While you're at it, read (or re-read) Eric Hoffer's The True Believer. Great stuff, especially for those of us who believe in something.