For almost 50 years, psychologists have been theorizing about and measuring religiosity essentially the way Gordon Allport did, when he distinguished between intrinsic and extrinsic religiosity. However, there is a historical debate regarding what this scale actually measures, which items should be included, and how many factors or subscales exist. To provide more definitive answers, we estimated a series of confirmatory factor analysis models comparing four competing theories for how to score Gorsuch and McPherson’s commonly used measure of intrinsic and extrinsic religiosity. We then formally investigated measurement invariance across U.S. Protestants, Irish Catholics, and Turkish Muslims and across U.S. Protestants, Catholics, and Muslims. We provide evidence that a five-item version of intrinsic religiosity is invariant across the U.S. samples and predicts less warmth toward atheists and gay men/lesbians, validating the scale. Our results suggest that a variation of Gorsuch and McPherson’s measure may be appropriate for some but not all uses in cross-cultural research.