An interesting documentary is opening in some major cities this weekend, and if you get the chance, I’d suggest that you check it out.

Titled “8: The Mormon Proposition,” the film examines the role the Church of Jesus Christ of Latter-day Saints (Mormons) played in helping pass Proposition 8 in California, which repealed marriage equality for same-sex couples.

Some statistics tell the story: Only about 2 percent of Californians are Mormons, but church members poured huge sums of money into the campaign – at least $22 million, including $3 million in the final week that came directly from Utah. Thanks to intervention by the church hierarchy, Mormons provided more than 70 percent of the budget of the pro-Prop 8 effort.

The money was used for an onslaught of ads and a well-coordinated ground campaign. As The New York Times put it, “The money financed a sophisticated media barrage that involved blogs, Twitter and YouTube videos, as well as scary (and, according to the movie, misleading) television ads, and an aggressive door-to-door campaign whose foot soldiers were instructed on how not to appear Mormon.”

The result was a narrow win for same-sex marriage opponents, 52 to 48 percent.

Of course, the Mormons were not the only group involved in the campaign. The Roman Catholic hierarchy and various fundamentalist groups jumped into the fray as well. But no other religious group could match the Mormons when it came to money, power and media relations efforts.

Religious groups have the right to speak out on social issues. But in this case, a wealthy, powerful (and mostly out-of-state) church poured unprecedented sums into an effort to write its theology into law and take away the rights of a group of people it does not like. To a lot of Americans, it just didn’t seem right.

Americans United Executive Director Barry W. Lynn appears in the film, making the point that when a powerful church is spending huge sums to influence everyone’s lives, the American public has the right to know that. Religious groups, he says, should not expect to be exempt from reporting requirements that are imposed on other organizations that engage in political activity.

California has reporting requirements that the church was required to follow. But many other states don’t, and in some cases, churches have demanded the right to be exempt from even minimal reporting requirements under the First Amendment. In those cases, it can be very difficult to follow the money if an aggressive fundamentalist group is backing an Astroturf campaign to elevate its theology into law.

It’s also not unreasonable to expect churches to abide by the law. In the case of the Mormon Church, that was not always reality. The church failed to report some non-monetary contributions in a timely manner and was recently fined $5,539 by the California Fair Political Practices Commission. (Sure, $5,550 is chicken feed to the Mormon hierarchy, but it sets an interesting precedent of accountability.)

California’s debate over same-sex marriage is far from over. A case is progressing in the federal courts arguing that California’s vote to ban same-sex marriage violates the U.S. Constitution’s guarantees of due process and equal protection.

A number of progressive religious groups have filed a brief in Perry v. Schwarzenegger, arguing that Proposition 8 enshrined right-wing theology into law and infringes on the religious liberty of those groups that support same-sex marriage. It’s an interesting argument that AU has made as well.

I’ve challenged Religious Right groups to provide even one secular reason for banning same-sex marriage. A few have tried, but what they offer isn’t remotely persuasive. Their rants about child-rearing and “traditional marriage” would be better put forward as arguments against divorce. And I’m not aware of any Religious Right organizations (or the Mormons, for that matter) campaigning to make divorce illegal.

You can see a trailer for “8: The Mormon Proposition” on the movie’s Web site. (If you don’t have the software to play it, try this site.) As I said, the documentary is playing in just a few major cities this weekend, but it will be available on DVD July 13.

Pull up some popcorn and take a look. You will be disturbed – but also enlightened.