With growing attention on the importance of values, beliefs and worldviews in shaping environmental outcomes, there remains little research on religion and sustainability transformations. We explored the impact of the Archbishop of Canterbury’s environmentally themed Lent Book 2020 “Saying Yes to Life” on environmental values, attitudes and behaviours of lay Christians. An online survey administered before and after reading the book assessed environmental values, New Ecological Paradigm (NEP), connectedness to nature and environmental behaviours, and collected open responses to questions about participants’ perceptions. Follow-up focus groups were also held to understand experiences of cognitive and behavioural change. Analysis of paired data revealed significant increases in environmental behavioural intentions after completing the book, especially for energy use, food and recycling. Some evidence for strengthening of NEP scores and connectedness to nature was also found. Open text responses corroborated with quantitative measures of behaviour change. Additionally, the majority of participants reported some form of reinforcement, confirmation, or further development or change in their beliefs and attitudes. This included a reduction in anthropocentric beliefs and greater appreciation of and obligation towards the natural world. Focus group discussions revealed diverse participant experiences, including having pre-existing theological beliefs affirmed, responding with new practical actions, connecting with spiritual experiences, and discovering systemic origins of unsustainability. Findings suggest potential for environmental interventions within religious contexts to shape mindsets, integrate theological views with environmental concerns, activate latent beliefs, and initiate and sustain pro-environmental behaviour. More intentional engagement with religion may facilitate transformative change for sustainability internally and externally, and across individual, organisational and societal domains.