Sexual and reproductive health policies often confront deeply rooted religious opposition that can limit their effectiveness. This paper examines how Zambia’s 2014 Comprehensive Sexuality Education (CSE) reform interacted with the rapid expansion of non-traditional Christian churches that strongly opposed the policy. Using a difference-in-differences strategy that exploits plausibly pre-determined variation in individuals’ exposure to these churches, we show that local religious environments critically conditioned the reform’s impact. Individuals more exposed to non-traditional churches experienced post-reform behavioral changes that ran counter to policy objectives, including lower use of modern contraceptives and higher rates of teen pregnancy. We examine two potential mechanisms: direct influence through religious messaging and indirect effects operating through the delivery of the curriculum in schools or students’ receptiveness to it.