People who campaign against religion in public life have an "intolerant faith position", Anglican and Roman Catholic church leaders have said.
Religion should be a public as well as private matter, says Dr Williams
Archbishop of Canterbury Rowan Williams and Catholic leader Cardinal Cormac Murphy-O'Connor say religion in society can be "radically inclusive".
Their comments come in a report by new religious think tank Theos.
The National Secular Society said religion in the public domain was often "divisive and discriminatory".
The report, called Doing God: A Future for Faith in the Public Square, argues against limiting religion to the private sphere.
In their joint foreword to the report, Dr Williams and Cardinal Murphy-O'Connor say: "Many secularist commentators argue that the growing role of faith in society represents a dangerous development.
"However, they fail to recognise that public atheism is itself an intolerant faith position.
"If we pay attention to what is actually happening in the United Kingdom and beyond, we will see that religiously-inspired public engagement need not be sectarian, and can in fact be radically inclusive."
The report comes after controversy over faith schools, with the government dropping proposals to make them accept pupils from non-religious backgrounds.
Meanwhile, atheist Oxford University professor Richard Dawkins's book The God Delusion has become a best-seller.
Theos director Paul Woolley said society was undergoing "rapid de-secularisation", and interest in faith was increasing in Western countries.
However, National Secular Society vice-president Terry Sanderson said religious intolerance in the UK came "almost exclusively from Christian evangelicals and minority faiths".
"The more Britain becomes a society in which competing religions jostle for power and religious observance continues to decline, the greater the case for a secular society where everyone is treated equally - regardless of religion or none - especially by the state," he said.