Two models seeking to explain the growth and decline of religious groups are prevalent in the literature. The religious market approach emphasizes the role of intergroup competition and in doing so focuses on religious switching. Another perspective emphasizes demographic mechanisms, particularly fertility. Research to date has not considered how switching and fertility interact as mechanisms of growth. Switching and fertility share a significant role in the growth trajectory of a religious group. Early success in gaining members through switching has an important long-term impact, which fertility alone cannot produce. The age of those switching into a group can also have significant consequences for the effects of fertility.