During the 2008 presidential campaign, rumors proliferated that Barack Obama was a Muslim who had been indoctrinated into militant Islam during childhood studies in a madrassa. The fact that the Democratic candidate had been a prominent and visible member of a Protestant church in Chicago for years somehow mattered not at all. The Obama campaign even created a Web site wholly devoted to answering conspiracy theories and smears.

Ultimately, though, it took a Republican in the form of Colin L. Powell to speak truth to fantasy. “He is not a Muslim, he’s a Christian. He’s always been a Christian,” the retired general and former cabinet secretary said on “Meet the Press.” “But the really right answer is, What if he is? Is there something wrong with being a Muslim in this country? The answer is no, that’s not America.”

Mr. Powell’s words echo now in the aftermath of last weekend’s massacre of six worshipers at a Sikh temple near Milwaukee. The narrative that has emerged in both media coverage and public discourse since then has been one of religious mistaken identity. It presumes that the killer, identified as a white supremacist named Wade M. Page, may have shot the Sikhs because he ignorantly believed they were Muslim.

Such a story line is accurate as far as it goes. Hundreds of times since the terrorist attacks on Sept. 11, 2001, Sikhs have been the victims of bias crimes. The perpetrators have invariably assumed that because Sikh men wear turbans and have beards they are Muslims, even specifically Taliban. How terrible it is that it has taken the slayings in Wisconsin to serve as a national teachable moment about the theology and practices of the Sikh religion.