We measure the impact of a return to meatless Fridays for English and Welsh Catholics on consumption behaviour, climate change mitigation, and religiosity. We find evidence of partial compliance with the reimposed obligation. We then measure the corresponding greenhouse gas reductions – which are nontrivial. We find no discernible impact on religiosity. We highlight the important role that religious regulations can play in achieving environmental sustainability.We identify a new source of low-cost greenhouse emission reductions, especially if this practice were to be reinstated by the Catholic Church at a global scale