Prior research has found mixed evidence for the long-theorized link between religiosity and pro-social behavior. To help overcome this divergence, we hypothesize that pro-social behavior is linked not to religiosity per se, but rather to the salience of religion and religious norms. We report on a field experiment that examines when auction participants will respond to an appeal to continue bidding for secular charitable causes. The results reveal that religious individuals are more likely than non-religious individuals to respond to an appeal "for charity" only on days that they visit their place of worship; on other days of the week, religiosity has no effect. Notably, the result persists after controlling for a host of factors that may influence bidding, but disappears when the appeal "for charity" is replaced by an appeal to bid for other reasons. Implications for the link between religion and pro-social behavior are discussed.