%K Demographic projection; religiosity; secularization; microsimulation; cohort effects %V 9 %A Ivan Puga-Gonzalez %A David Voas %A Lukasz Kiszkiel %A Rachel J Bacon %A Wesley J Wildman %A Konrad Talmont-Kaminski %A F LeRon Shults %X This article presents a microsimulation that explores age, period, and cohort effects in the decline of religiosity in contemporary societies. The model implements a well-known and previously empirically validated theory of secularization that highlights the role of “fuzzy fidelity,†i.e., the percentage of a population whose religiosity is moderate (Voas 2009). Validation of the model involved comparing its simulation results to shifts in religiosity over 9 waves of the European Social Survey. Simulation experiments suggest that a cohort effect, based on weakened transmission of religiosity as a function of the social environment, appears to be the best explanation for secularization in the societies studied, both for the population as a whole and for the proportions of religious, fuzzy, and secular people. %J Journal of Religion and Demography %N 1-2 %L discovery10163198 %I Brill %D 2022 %O This version is the author accepted manuscript. For information on re-use, please refer to the publisher’s terms and conditions. %T Modeling Fuzzy Fidelity: Using Microsimulation to Explore Age, Period, and Cohort Effects in Secularization %P 111-137