The former leader of the Church of England, now Lord Williams of Oystermouth, said that the time of habitual worship is over .
He predicted that a further decline of widespread faith is likely in the future.
His comments yesterday come after the Prime Minister was criticised for saying the UK should be "more confident about our status as a Christian country" and "more evangelical" about faith.
David Cameron's comments prompted fury from secular and atheist groups.
It led to Liberal Democrat leader Nick Clegg, an atheist, calling for the separation of church and state in England.
A poll for the newspaper also found that while more than half the public regard Britain as a Christian country, the majority of practising Christians are afraid to express their beliefs.
Lord Williams, who is now Master of Magdalene College, Cambridge, said that while Britain's "cultural memory is still quite strongly Christian", it is "post-Christian" in that habitual practice is not taken for granted.
He said: "A Christian nation can sound like a nation of committed believers, and we are not that."
Underlining the UK's changing relationship with religion, he added: "A Christian country as a nation of believers? No. A Christian country in the sense of still being very much saturated by this vision of the world and shaped by it? Yes."