The coalition's bid to legalise same-sex marriage will be compared to Nazism by a Catholic bishop later.
Mark Davies, the Bishop of Shrewsbury, will reportedly draw a parallel between the attempt to place gay marriage on an equal legal footing with heterosexual couples and the actions of 20th century authoritarian regimes in his Christmas Mass sermon.
"These inhuman ideologies would each challenge in the name of progress the received Christian understanding of the sanctity of human life and the family," a preview of the speech reported by the Mail newspaper stated.
"Winston Churchill, Britain's wartime prime minister, a man without clear, religious belief, saw in this deadly struggle nothing less than the defence of Christian civilisation.
"Few of our political leaders today appear to glimpse the deeper issues when the sanctity of human life and the very identity of marriage, the foundation of the family, are threatened."
The Bishop of Shrewsbury is expected to accuse David Cameron of having agreed to the reform "without any serious consideration".
He is set to add: "The British people have reason to ask on this night, where is such progress leading?"
His sermon is likely to generate outrage from gay rights activists, who hope the coalition will make good on its promise to legalise same-sex marriages so gay couples can get married as soon as next summer.
The prime minister is using his Christmas message to demonstrate men of the cloth do not have a monopoly on religion, however.
David Cameron said the festive season gives Brits an opportunity to look back on an "extraordinary" 2012, before adding: "Christmas also gives us the opportunity to remember the Christmas story – the story about the birth of Jesus Christ and the hope that he brings to the countless millions who follow him.
"The Gospel of John tells us that in this man was life, and that his life was the light of all mankind, and that he came with grace, truth and love.
"Indeed, God's word reminds us that Jesus was the Prince of Peace."
Cameron's message concluded with a tribute to the armed services personnel serving in Afghanistan this Christmas.
"When we are celebrating with family and friends, they and many others are all working on our behalf and deserve our thoughts and appreciation," he added.
"So however you celebrate this time of year, it is my hope and prayer that you have a happy and peaceful Christmas."
His sentiments were echoed by Labour leader Ed Miliband, who acknowledged that 2012 had been "a particularly challenging year" for UK soldiers serving overseas.
"But wherever you are today – whether in a base in Afghanistan, on a Royal Navy vessel, serving as part of a Royal Air Force operation, or serving at home – you can stand proud," Miliband added.
"You are protecting the interests of the people of Britain. You are working for a better future for the people of Afghanistan. And you can be proud to be part of a long and honourable history of men and women who have served our country with honour, selflessness and bravery."