Supporters attend an Oct. 21 march in Sydney in support of a referendum that would legalize same-sex marriage in Australia. (Lisa Maree Williams/Getty Images)

A referendum to legalize same-sex marriage has been seized on by church leaders in this once overwhelmingly Christian country as a battle for the soul of Australia.

With religious belief in steady decline, the country's Christian leaders are saying that the sanctity of marriage as well as the faith itself are under attack by this measure.

Although 70 percent of Australians describe themselves as religious, polls suggest a majority also believe same-sex couples should be allowed to marry — a position in direct contradiction to the teachings of many church leaders.

Advocates for the new measure maintain that little will change, apart from letting people who love each other have weddings. Gay couples in Australia have been substantially treated like married couples by the federal government since 2008, when a law extended their access to social security and tax benefits.

Australians have until Nov. 7 to vote by returning a yes-or-no form by mail that asks whether the law should be changed to allow same-sex marriage. Prime Minister Malcolm Turnbull, caught between the global movement to grant gay and transgender people more rights and opposition from his center-right party's conservative wing, has said he will respect the results of the vote.

Many religious leaders see same-sex marriage as likely to weaken churches and lead to a more secular society. Led by Australia's two biggest churches, the Catholics and the Anglicans, a well-organized and funded campaign has been trying to mobilize supporters.

The plan doesn't seem to be working. So far, 12 million votes have been cast, a return rate of 75 percent, which experts say is a good sign for the "yes" campaign. The more people who vote, the more the outcome is likely to reflect opinion polls that show a majority want the law changed, analysts say.

"It is a moment where the influence of Christianity hits a threshold," said Peter Sherlock, president of the Melbourne-based University of Divinity. "It shows that the churches are further out of step with how men and women relate to each other and how society sees marriage."

Christianity has deep roots in Australia's history and identity since contact with the Western world. According to historical records, six days after the first British fleet arrived in Sydney's harbor in 1788, an evangelical priest, the Rev. Richard Johnson, held a service under a tree on a grassy hill.

Johnson, who also served as a magistrate, wanted the unruly penal colony to become a model of Christian respectability. He built Australia's first church and worked hard to convert the aboriginal population. Over the following century and a half, his successors made religion central to Australian society.

Today, the Lord's Prayer is still recited in the Australian Parliament when it meets, and most top schools are controlled by religious orders or churches, which receive generous tax breaks. Bishops and archbishops have high social status.

Their influence is threatened by changing attitudes toward religion — a trend evident across most of the Western world.

The decline in religiosity in Australia is clear in polls. In 1966, 88 percent of Australians described themselves as Christians, according to government census figures. By 1991, the figure had slipped to 74 percent. By 2016, it was 52 percent.

Australia also is home to Muslims, Hindus, Jews and followers of other faiths.

While expressing respect for gay couples' love, religious officials assert that if the referendum passes, religious institutions, including churches, hospitals and schools, could come under attack.

Religious schools could be forced to teach that male and female sexuality is fluid, or business people could be sanctioned for refusing to provide services for gay couples, they say.

The case of Colorado baker Jack Phillips has been cited as a cautionary tale. The American was found to have violated state law by refusing to create a cake to celebrate a gay wedding. Phillips said that doing so would violate his religious beliefs. The case is now before the U.S. Supreme Court, and the Trump administration has sided with the baker.

"This is a monumental decision for the Australian public," the Anglican archbishop of Sydney, the Most Rev. Glenn Davies, wrote in a letter urging parishioners to vote. "We know from recent experiences in the United Kingdom and North America that the ramifications of such a change are profound."

"Marriage is taking quite a beating right now," Sydney's Catholic archbishop, Monsignor Anthony Fisher, wrote last month in the Daily Telegraph, a Sydney tabloid. "Faithophobic slurs are now all too common."

The vote also has become entwined in a power struggle within the governing Liberal Party. Former prime minister Tony Abbott, who was removed from power two years ago by Turnbull, has emerged as a leading advocate of the "no" case. Turnbull has said he will vote for change.

Abbott appears to be using the debate to appeal to conservatives within his own party and put pressure on his much more liberal successor.

"If you're worried about religious freedom and freedom of speech, vote no," Abbott recently told reporters.

The issue has split the Abbott family. One of the politician's sisters is gay and says she will marry her partner in the British consulate in Sydney if the vote fails. One of Abbott's daughters recently declared her support for the "yes" campaign on Instagram.

The debate has been mostly peaceful, although Abbott was head-butted while walking down a Tasmanian street by a man wearing a "vote yes" button on Sept. 21.

The man, a disc jockey who uses the stage name Funknukl and was charged with assault, later told reporters he disliked Abbott's character and didn't specifically attack him over same-sex marriage. Nonetheless, the incident fueled feelings among "no" advocates that they are under siege from secular activists.

A week later, a study published by a University of Queensland sociologist said that opponents of equal rights for same-sex couples scored lower on a common test of cognitive ability.

The academic, Francisco Perales, said he received "all sorts of abuse" in response to the article, which was shared over 10,000 times on Facebook. Others expressed gratitude for what they described as his bravery.

"Several mentioned it meant a lot to them, given the abuse and moral scrutiny they are having to go through during the campaign as LGBT (lesbian, gay, bisexual and transgender) families," he said in an email.