This article distills theoretical arguments that I advance in Foreigners and Their Food, arguments relevant to a wide range of religious studies scholars. In addition, it makes the case for comparison as a method that sheds light not only on specific comparands and the class of data to which they belong but also on the very boundaries which the comparison transgresses. Through a comparison of Latin Christian and Shiʿi Islamic discourse about the impurity of religious foreigners, I illustrate methods by which religious authorities develop and transmit conceptions of foreigners. I then analyze this case study using Oliver Freiberger’s “Elements of a Comparative Methodology” while assessing the strengths and limitations of Freiberger’s methodi...