Cameron said the bravery of British soldiers in Afghanistan reminds him of John's description of Jesus, The Guardian reported.
"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," Cameron said. "Indeed, God's word reminds us that Jesus was the Prince of Peace."
On a visit to Afghanistan last week, Cameron joined British soldiers for carols.
In his message, Cameron also touched on Queen Elizabeth's Diamond Jubilee and the Olympics. He said Britons "punched way above our weight in the medals table."
British politicians tend to treat religion as a private and personal matter. Even Prime Minister Tony Blair, a devout Christian who became a Catholic after leaving office, tended to avoid public expressions of faith.
In a 2008 interview, Cameron, asked about his religious views, quoted London Mayor Boris Johnson. Johnson once said his faith was like the reception for FM radio in the Chiltern hills: "It sort of comes and goes."
"That sums up a lot of people in the Church of England," Cameron continued.
"We are racked with doubts, but sort of fundamentally believe, but don't sort of wear it on our sleeves or make too much of it. I think that is sort of where I am."