  We propose a simple model to describe the dynamics of religious affiliation.
For such purpose, we built a compartmental model with three distinct
subpopulations, namely religious committed individuals, religious noncommitted
individuals and not religious affiliated individuals. The transitions among the
compartments are governed by probabilities, modeling social interactions among
the groups and also spontaneous transitions among the compartments. First of
all, we consider the model on a fully-connected network. Thus, we write a set
of ordinary differential equations to study the evolution of the
subpopulations. Our analytical and numerical results show that there is an
absorbing state in the model where only one of the subpopulations survive in
the long-time limit. There are also regions of parameters where some of the
subpopulations coexist (two or three). We also verified the occurrence of two
distinct critical points. In addition, we also present Monte Carlo simulations
of the model on two-dimensional square lattices, in order to analyze the impact
of the presence of a lattice structure on the critical behavior of the model.
Comparison of the models' results with data for religious affiliation in
Northern Ireland shows a good qualitative agreement. Finally, we considered the
presence of inflexible individuals in the population, i.e., individuals that
never change their states. The impact of such special agents on the critical
behavior of the model is also discussed.
