Prop 8 is often cast as a battle between gay rights groups and religious groups (Mormons, evangelicals, Catholics, etc). But I’m reminded by this note from a leader of the pro-gay marriage forces in Massachusetts that in her state the pro-marriage clergy played a crucial role — and in California, not so much.
The Reverend Anne C. Fowler, President, Religious Coalition for the Freedom to Marry, Massachusetts, wrote:

The Massachusetts Religious Coalition for the Freedom to Marry began as a small group of signers in 1997 or 98. By 2003, when the Goodridge case [which considered whether gays had the right to marry in Massachusetts] was being argued, we became proactive and began gathering signatures to our statement of support for marriage equality. By 2007 we had 1,000 clergy supporters and 4,000 Roman Catholic lay signers.

We held worship services at times of celebration and crisis as equality became the law and as the ballot initiative made its way through various Constitutional Conventions and court challenges. We organized and trained clergy and their congregations in lobbying their legislators, and talked with legislators personally. We were highly visible in clergy regalia at all public demonstrations, vigils, and rallies for equality.
It seemed that California was awash in money, but from what I’ve heard the secular campaign did not engage clergy in an effective way. The staff person for Faith Equality CA got in touch with me in the summer when Proposition 8 surfaced as an election issue. We gave her the best advice we could. It was apparent that they were not organized at that time.
From everything I have heard, the secular No on Prop 8 campaign in California did not mobilize the progressive faith community. Political organizers do not have the status or authority of clergy, nor do they spend their days with “the people in the pews” : folks who may not be activists themselves but can be persuaded that their faith supports justice and equality for all, including same-sex couples who wish to marry. Political organizers can’t convince congregations that marriage equality will not force clergy to marry couples against their conscience. Only clergy can do that.
I have read news articles awarding credit or blame for the passage of Prop 8 variously to the Mormon Church, African American voters influenced by their clergy, or the Roman Catholic bishops. Could this not have been anticipated by the No on Prop 8 Campaign? Will every state have to learn from scratch the lesson of the critical role of religious leaders?

She noted too that even in Massachusetts this lesson can be forgotten quickly. At a recent anti-Prop 8 rally there, no clergy spoke.