The Archbishop of Canterbury has said David Cameron is "right" to state that the UK is a "Christian country".
The Most Rev Justin Welby wrote in his blog that it was a "historical fact (perhaps unwelcome to some, but true)" that UK law, ethics and culture were based on its teachings and traditions.
He called the criticism of Mr Cameron by "atheist protesters" over his remarks "baffling".
Opponents argued that the prime minister's intervention was "divisive".
Prior to Easter, Mr Cameron wrote in the Church Times that people in the UK should be "more confident about our status as a Christian country".
This prompted a group of 50 public figures to write a letter to the Daily Telegraph insisting that the UK was "a non-religious" and "plural" society and that to claim otherwise "fosters alienation and division".
It was "wrong to try to exceptionalise" the contributions of Christianity to national culture, they added.
In response, Mr Cameron's Conservative cabinet colleague Iain Duncan Smith called the claims "absurd", while another, Dominic Grieve, said atheism had not made "much progress" in the UK.
Writing in his blog, Dr Welby, said: "Judging by the reaction (to Mr Duncan Smith and Mr Grieve), anyone would think that the people concerned had at the same time suggested the return of the Inquisition (complete with comfy chairs for Monty Python fans), compulsory church-going and universal tithes."
He added: "It's all quite baffling and at the same time quite encouraging. Christian faith is much more vulnerable to comfortable indifference than to hatred and opposition. It's also a variation on the normal 'Sword and Grail discovered' stuff that seems to be a feature of Easter week news.
"Yet the prime minister and other members of the government have not said anything very controversial. It is a historical fact (perhaps unwelcome to some, but true) that our main systems of ethics, the way we do law and justice, the values of society, how we decide what is fair, the protection of the poor, and most of the way we look at society... All have been shaped by and founded on Christianity.
"Add to that the foundation of many hospitals, the system of universal schooling, the presence of chaplains in prisons, and one could go on a long time. Then there is the literature, visual art, music and culture that have formed our understandings of beauty and worth since Anglo-Saxon days."
The 2011 census found 59% of people in England and Wales said they were Christians - down from 72% a decade earlier.
Dr Welby, who has been Archbishop of Canterbury since 2012, said: "It is clear that, in the general sense of being founded in Christian faith, this is a Christian country.
"It is certainly not in terms of regular church-going, although altogether, across different denominations, some millions attend church services each week. Others of different backgrounds have also positively shaped our common heritage.
"But the language of what we are, what we care for and how we act is earthed in Christianity, and would remain so for many years even if the number of believers dropped out of sight (which they won't, in my opinion)."
Criticising the signatories of the letter to the Telegraph, who included the writers Philip Pullman and Sir Terry Pratchett, Dr Welby said: "The atheist protesters are wrong to argue that expressing confidence in the country's Christian identity fosters alienation and division in our society.
"Indeed, it is significant that non-Christian faith leaders - among them Anil Bhanot of the Hindu Council UK, Farooq Murad of the Muslim Council of Britain and Lord Indarjit Singh of the Network of Sikh Organisations - have spoken out in support of Mr Cameron."
He added: "So why the fuss? As I say, for all of us, in the church, of Christian faith, of any tradition or set of beliefs, history makes for some uncomfortable reading. Its facts are awkward for all of us, but it is no use pretending they do not exist. The PM is right on this."