Why do similar modern nations accord religion different roles in their public institutions? This paper engages this question by examining trends in religious instruction in the public schools of the United States and Australia from 1850 to 1950. I find that American education secularized farther and faster than Australian education because of its decentralized system of educational administration. In the United States, decentralized educational administration facilitated challenges to religious exercises by religious minorities, fostered professional development among educators, and allowed novel educational practices oriented in new educational theories rather than religion to spread. In Australia, by contrast, centralized state control over education insulated majoritarian religious exercises from minority criticism, suppressed professional development, and helped maintain traditional educational practices that sustained religious instruction. The state thus has both mediating and constitutive effects on secularization, a finding which opens new directions for research into the dynamics of secularization.