This paper explores the relationship between religious affiliation and support for a constitutional amendment banning same-sex marriage, and how this relationship changes over the course of the campaign. There is a demonstrable connection between religious beliefs and partisan and ideological preferences in the United States (Patrikios 2008; Campbell 2007; Layman 2001). Using Proposition 8 in California (2008) as my primary case study, I consider how religion shapes voter behavior specifically in same-sex marriage ballot propositions and how moral policy decisions most clearly expose the link between religious beliefs and ideological preferences. I find that the predictive capacity of religion with respect to Proposition 8 vote choice increases for the non-Christian vote over the course of the 2008 campaign cycle. In contrast, religion produces stable voting preferences among Christian voters throughout the campaign. From these results, I conclude that the religion effect on non-Christian Proposition 8 vote choice is magnified when accompanied by campaign mobilization, same-sex marriage political salience and the effect of public opinion. My analysis shows that the religion effect, coupled with California’s political opportunity structure, stabilizes Christian Proposition 8 vote preference, while increasing opposition amongst non-Christian Proposition 8 voters over the course of the campaign (Soule 2004). While campaign effects narrowed the gap between “yes” and “no” votes on Proposition 8, California’s ballot initiative system ultimately favors moral policy outcomes that resonate with deeply held religious beliefs (Haider-Markel and Meier 1999). Drawing on survey data from the Public Policy Institute of California, I argue that same-sex marriage electoral outcomes are a product of timely campaign mobilization tactics, the unique California voter initiative system, and religion effects that simultaneously stabilize voting patterns among Christian voters and provide opportunities for non-Christian voters to mobilize against same-sex marriage bans.