Why does religion play a central role in some societies? Rulers have historically used religion to legitimize their power, which incentivized them to embed religion into institutions. This institutionalization of religion thus may explain why religion persists despite modernization. Using data across 1265 premodern societies and 176 countries, we provide evidence supporting divine legitimization and the resulting institutionalization of religion. For identification, we exploit exogenous variation in the incentives to employ religion for power purposes. We document two implications: countries that relied more on divine legitimization are more autocratic today and their populace more religious.