A poll by ComRes has found that nearly eight out of 10 churchgoers (79%) believe that the Prime Minister's pledge is worthless.
Of the 569 churchgoers surveyed, 86% said that even if the Government changes the law to exempt religious buildings this will be overturned by the European Court of Human Rights.
Colin Hart, campaign director of C4M, which commissioned the ComRes poll, said the Government's plans to legalise gay marriage were "ill thought out and undemocratic".
"Churchgoers simply don't believe the assurances from the Government that these changes will not be forced on churches," he said.
He pointed to recent reports of leaked emails in Scotland which indicated that clergy could be forced to carry out gay weddings against their beliefs if the Government fails to change the Equalities Act.
The Prime Minister has said previously that the plans would only affect "what happens in a register office, not what happens in a church".
The ComRes poll found that nearly two-thirds of churchgoers (63%) feel that the PM is "intolerant in his reaction to opposition to his plans for same-sex marriage".
Worryingly for Mr Cameron, the survey found 58% churchgoers were less likely to vote for the Conservative leader at the next general election.
Forty-three per cent of Christians said they were less likely to vote Lib Dem as a result of the gay marriage plans, with just 2% saying they were more likely. Sixty-five per cent of churchgoers said their views of the Lib Dem leader had got worse because of the proposals.
Ed Miliband the Labour leader, fared slightly better with nearly three in ten (27%) saying they were less likely to vote Labour at the next election and 8% saying they were more likely. Thirty-seven per cent said their opinion of the Labour leader had worsened.
Fourteen per cent said the gay marriage proposals would make them more likely to vote for UKIP, but the same percentage said less likely.
While the main political parties are set to experience a dip in Christian support, a ComRes poll in May revealed weak support within the gay community, with only 39% of gay people saying they felt the change was a priority.
The same poll revealed that just under one in five (19%) of gay people thought Mr Cameron was supporting gay marriage because of his convictions.
A February ComRes poll of the general public found that 70 per cent of people back the current definition of marriage.
Mr Hart said: "Those pushing forward these proposals are increasingly looking out of touch with ordinary members of the public who see this as an unnecessary distraction.
"How much longer can the Government ignore the overwhelming weight of the arguments against these plans and the growing public opposition to them? It's time to ditch them."