Entering a debate triggered by David Cameron's declaration before Easter that Britain should be "more confident about our status as a Christian country" and "more evangelical" about faith, Williams said that Britain was "post-Christian", but not "non-Christian".
Speaking in an interview with the Sunday Telegraph, Williams – now Lord Williams of Oystermouth – said it was important to "pick your way quite carefully" in the debate about the nation's relationship with Christianity.
"If I say that this is a post-Christian nation, that doesn't mean necessarily non-Christian. It means the cultural memory is still quite strongly Christian. And in some ways, the cultural presence is still quite strongly Christian. But it is post-Christian in the sense that habitual practice for most of the population is not taken for granted.
"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."
Professor Jim Al-Khalili, president of the British Humanist Association, was one of more than 50 public figures who signed a letter to the Daily Telegraph criticising Cameron's comments about Britain being a Christian country. They said that, apart from in a "narrow constitutional sense", there was no evidence to justify labelling Britain as a Christian state and that to assert otherwise ran the risk of fostering "alienation and division".
In his interview, when pressed for a yes/no response to the question about whether Britain was a Christian country, Williams replied: "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."
He also said that, in the future, awareness of Christianity might decline. "Given that we have a younger generation who now know less about this legacy ... there may be a further shrinkage of awareness and commitment."
But this could also lead to people discovering Christianity afresh, he said. "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 ... in school visits. There is a curiosity about Christianity."