Britain is in 'a post-Christian era'.
Lord Williams of Oystermouth, who stood down as leader of the Church of England in 2012, said that the time of habitual worship is over and that a further decline of faith is likely in the future.
His comments, in the Sunday Telegraph, come after the Prime Minister was criticised for saying the UK should be "more confident about our status as a Christian country".
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 across most of the population is not taken for granted.
He said: "A Christian nation can sound like a nation of committed believers, and we are not that. Equally, we are not a nation of dedicated secularists.
"I think we're a lot less secular than the most optimistic members of the British Humanist Association would think."
Underlining Britain'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."
Asked whether Britain will lose its faith altogether, he said: "Given that we have a younger generation now who know less about this legacy than people under 45, there may be a further shrinkage of awareness and commitment.
"The other side is that people then rediscover Christianity with a certain freshness, because it's not 'the boring old stuff that we learnt at school and have come to despise'. I see signs of that, talking to youngsters here at Magdalene and in school visits.
"There is a curiosity about Christianity."
The Sunday Telegraph's poll, which surveyed 2,000 people online, found that 56% of people regard Britain as Christian, rising to 60% among men and 73% among over-65s.
Former Archbishop of Canterbury Dr Rowan Williams <B