gt; 1,700), we investigated whether costly behaviours are more effective in promoting trust when integrated within a religious rather than secular context using the pilgrimage to Santiago de Compostela as a costly display of commitment. First, we show that pilgrims base their pilgrim identity on physical effort (Studies 1A and 1B).Next, in three pre-registered experiments (Studies 2–4) with the Spanish population, we compared the trustworthiness of people posting on Facebook about their participation in a religious pilgrimage and a secular pilgrimage/hike with various control posts. The results showed that pilgrims/hikers are perceived as more trustworthy than non-pilgrims and that long-distance pilgrims are perceived as more trustworthy than short-distance pilgrims.Moreover, these effects are stronger when the pilgrimage is framed in a religious context compared to a secular context. Our research highlights the key role of religion in the costly signalling of commitment.