I think the Mormonism answer might be a close call. Salt Lake City has a population of about 194,000, it is in a county that is about 49% Mormon, and Wikipedia says that most of the county's Mormons reside in the southern part (where Salt Lake City isn't). Thus, a generous estimate for the number of Mormons in Salt Lake City would be 95,000. Consider instead Provo, Utah, which has a population of about 116,000 and is in a county that is about 82% Mormon. Assuming that Provo itself is 82% Mormon gives about 95,000 Mormons in it, as well.