In a 1907 essay title Christian Science, Mark Twain explains the normal psychology of group identity, and spins it into a plea for moral humility:

“Let us consider that we are all partially insane. It will explain us to each other, it will unriddle many riddles, it will make clear and simple many things which are involved in haunting and harassing difficulties and obscurities now…

"That is a simple rule, and easy to remember. When I, a thoughtful and unbiased Presbyterian, examine the Koran, I know that beyond any question every Mohammedan is insane; not in all things, but in religious matters. When a thoughtful and unbiased Mohammedan examines the Westminster Catechism, he knows that beyond any question I am spiritually insane. I cannot prove to him that he is insane, because you never can prove anything to a lunatic–for that is part of his insanity and the evidence of it. He cannot prove to me that I am insane, for my mind has the same defect that afflicts his. All democrats are insane, but not one of them knows it; none but the republicans and mugwumps know it. All the republicans are insane, but only the democrats and mugwumps can perceive it. The rule is perfect; in all matters of opinion our adversaries are insane. When I look around me I am often troubled to see how many people are mad [gives long list]… This should move us to be charitable toward one anothers lunacies.”

[Thanks to Bo Ledbetter for sending me this quote]