(RNS) As a federal appeals court in California heard opening arguments in a landmark gay marriage case on Monday (Dec. 6), a rare coalition of religious leaders released a public letter promoting marriage as “the faithful union of one man and one woman.”

Leaders of Catholic, evangelical, Orthodox Jewish, Mormon, Eastern Orthodox, Pentecostal and Sikh communities, as well as new conservative Lutheran and Anglican denominations, signed “The Protection of Marriage: A Shared Commitment.”

“As religious leaders across different faith communities, we join together and affirm our shared commitment to promote and protect marriage as the union of one man and one woman,” the brief letter states.

Such interfaith alliances were once rare, but are becoming more common as conservatives seek allies in culture-war battles over gay marriage, abortion, and embryonic stem cell research. In 2008, for example, Catholic and Mormon leaders allied in a successful campaign to ban gay marriage in California.

Archbishop Timothy Dolan of New York, the new president of the U.S. Catholic Bishops Conference, said the “broad consensus reflected in this letter — across religious divides — is clear: The law of marriage is not about imposing the religion of anyone, but about protecting the common good of everyone.”

The letter comes as courts and state legislatures are wrestling with how to define marriage. While polls show a growing number of Americans — especially youth — favor allowing gays and lesbians to wed, prominent religious leaders are lobbying hard to limit marriage to heterosexual unions.

Dozens of protesters clashed outside a federal courthouse in San Francisco on Monday, according to the Los Angeles Times, as the U.S. 9th Circuit Court of Appeals heard opening arguments in a case centered on California’s gay marriage ban.

Voters approved a referendum in 2008 that amended the state’s constitution to define marriage as the union of one man and one woman. But a federal district judge earlier this year ruled that the ban violates the constitutional rights of gays and lesbians.

The U.S. Catholic bishops conference challenged that reasoning on Monday in a Q&A released with the open letter. “No one has the right to change and redefine the nature of marriage,” the bishops said.

The interfaith letter will be sent to the country’s nearly 19,000 Catholic parishes where it will be inserted in church bulletins, according to the bishops.