How do people’s religious beliefs and behaviors change over the course of adulthood? Previous research found rapid decreases in religiosity during young adulthood and rebounds in middle and late adulthood. However, secularization trends—if not accounted for—can bias or obscure age-graded changes in religiosity. Using longitudinal data from over 14,000 Dutch participants aged 16 to 101 years, we disentangled secularization trends from developmental changes in religiosity. Controlling for secularization, we found no evidence for age-graded declines in religiosity among young adults but lifelong increases in religiosity. These increases were most pronounced during middle to late adulthood, consistent with theories that emphasize the self-transcendent focus of this life stage. College-educated individuals were generally less religious and experienced less pronounced age-graded increases in their religious beliefs. These findings must be understood in the context of secularization trends as indicated by significant decreases in religiosity among people of all demographic groups.