The potential benefits of integrating vernacular religious beliefs into biodiversity conservation and sustainability actions have been emphasized widely, while specific examples and the synergies between religious communities and conservationists in particular spaces deserve more attention. The manifestations of vernacular religions in territories devoted to sustainability, such as Biosphere Reserves (BRs), are a particularly rich area for study. While La Campana-Penuelas BR has long been recognized as a territory where local religion is an important feature, greater awareness of this and of the potential link to sustainability actions is still lacking, as is awareness of the potential of the BR to become a Multi-Internationally Designated Area. Here we report on the diversity of popular religious activities within the BR. Possible synergies between popular religion and sustainability are based on the strong organizational capacities of local communities, and on an ethical vision of social justice that encompasses both human relations and attitudes towards the non-human beings within the biosphere.