The proportion of Hispanics who are Catholic remains unclear, partly because of varying survey methods and limited understanding of how these variations affect estimates of Hispanic religious identification. We compare 12 national surveys conducted since 1990. Language use strongly predicts religious identification among Hispanics—more strongly than other indicators of assimilation—and evidence suggests English-only interviewing inflates Protestant identification. Additionally, identifying Hispanics through ancestry questions may inflate Catholic identification. We also explore effects of sampling bias, noncoverage bias, and weighting on religious identification. Analyses suggest that poststratification weighting is advisable, particularly for language use. However, weighting cannot fully substitute for extensive coverage of subpopulations such as recent immigrants and Spanish-only speakers. We conclude that 70 percent or slightly more is a reasonable estimate of the proportion of adult Hispanics who are Catholic, and 20 percent a reasonable estimate of the proportion who are Protestant or other Christian.