A little while ago I telephoned Professor Antony Flew at his home in Reading. The philosopher once described as “the world’s most famous atheist” was having his lunch. Could I call back later? When I did, however, the great man was not exactly forthcoming. “Professor Flew,” I began, “I wonder if you would be willing to be interviewed for the New Statesman?” “I am old and decrepit,” replied the prof, “but my mind is still sharp. So my answer is no.” Click, brrrr.

The reason for Flew’s refusal, and his brevity, was not some curious dislike for the NS. The answer lies in the designation above. He may once have been described as “the world’s most famous atheist”, but no more. Flew caused a stir – made news around the world, in fact – in 2004, when it was reported that he now believed in God. There had already been rumours of his “conversion” three years previously, which he denied with a response titled: “Sorry to disappoint, but I’m still an atheist!” This time they were confirmed.

New scientific discoveries persuaded him, he said, “that intelligence must have been involved” in producing life. He later backtracked on the reasons for his change of heart, saying he had been misled by the evidence he’d been presented with, a statement that ­attracted some derision in humanist and philosophical circles. Which is why, I suspect, that at the age of 86, Flew doesn’t want to go into all this in depth again.

He does, however, still believe in God – or, in his case, god. For Flew had become not a Christian, but a deist, a distinction the British Humanist Association correctly noted on its website, where it continued for a while to list him as a “distinguished supporter” with the regretful rider: “Professor Flew has recently become a deist. Nevertheless, we would like to thank him for his many years of support.”

Flew was no more sympathetic to the revealed religions of the Book, with their “monstrous Oriental despots” of gods, as he called them, than before. He had simply come to the conclusion that, at the very least, there was probably some kind of “first cause”; and that this, rather than an interventionist deity presiding over an afterlife, was what he meant by “god”.

Most people have probably never heard of the term deism, or, if they had, would fail to distinguish it from theism. The confusion would be understandable given that the two terms’ derivations differ merely in that deist comes from the Latin deus and theist from the Greek theos, and that both mean “god”. The two are very different, however.

Deists believe in a god who created but does not intervene in the universe. That god, however, does not have to be anything more than an entity that set creation in motion. It does not give you the anthropomorphised deity to whom many believers pray, nor any of the trappings and beliefs that we associate with religion.

Theism, on the other hand, implies belief in the God of the Abrahamic religions, who remains present to and active within the world at the same time as transcendent over it.

But, from the Enlightenment onwards, the influence of deism has been vast. Many of America’s Founding Fathers, including Thomas Jef­ferson and Benjamin Franklin, were deists, as were the philosophers Rousseau and Voltaire and the English radical pamphleteer Tom Paine. The precise nature of Flew’s deism is a matter of considerable controversy. Some allege that the philosopher has been taken advantage of, and that his 2007 book There Is a God was mainly the work of his American co-author, Roy Varghese, although Flew vigorously denies this. Nevertheless, many felt that the book lacked the coherence and style of his earlier works, such as God and Philosophy and his essay “The Presumption of Atheism”, and did not show the brilliance of a mind known to generations of undergraduates. A New York Times reviewer summed up the mood of the new book’s detractors: “I doubt thoughtful believers will welcome this volume. Far from strengthening the case for the existence of God, it rather weakens the case for the existence of Antony Flew.”

The deism of the Founding Fathers, however, was significantly stronger than that which can be ascribed to Flew with con­fidence. Theirs was that of a natural religion, one that was not revealed to Middle Eastern prophets but could be arrived at by reason. The laws of nature must have been designed, goes the argument, hence there must be a designer, and the concept of natural rights (which so permeates the United States constitution) is embodied in his creation.

Any belief in scriptural authenticity or an ­afterlife is not entailed, although many of these deists were close enough to religion for it to be queried today whether they were, in fact, not deists but rationalist theists. Benjamin Franklin was typical of those who took this approach. “As to Jesus of Nazareth,” he wrote to the president of Yale University in 1790, “I think the system of morals and his religion, as he left them to us, the best the world ever saw or is likely to see; but I have, with most of the present dissenters in England, some doubts as to his divinity; though it is a question I do not dogmatize upon, having never studied it, and think it needless to busy myself with it now, when I expect soon an opportunity of knowing the truth with less trouble.”

If Franklin’s words strike a chord with many, including those who think of themselves as being Christians, perhaps that is no surprise. According to Tom Wright, the Bishop of Durham and a world-renowned New Testament scholar, most of them are in fact deists – whether they know it or not. “I think that almost all ‘ordinary English’ people – and a good many others, too – simply take a deist framework of thought for granted and when they hear the word ‘god’ that’s what they are thinking of,” he tells me. “The fact that there is a major difference between deism and the three Abrahamic religions is not just news to most; it is incomprehensible when the ‘news’ is told them.”

Wright’s analysis certainly fits with the vague professions and low-level observance that characterise the popular image, and often the reality, of English churchgoing (as opposed to the more rigid theologies and greater demands placed on followers of, say, Catholicism and Islam). And if it is correct, it is of far greater significance than the decision of one particular atheist, however famous, to join them. C S Lewis was once himself a deist, until he took a journey to Whipsnade Zoo in the sidecar of his brother’s motorbike, at the end of which he found he had become a Christian. But his later words, subsequent to his final conversion, are a stern rebuke to any Christian who fails to affirm the divinity of Christ, or thinks of him merely as a great teacher; for they have in fact lapsed into deism. “A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher,” wrote Lewis. “He would either be a lunatic – on a level with the man who says he is a poached egg – or else he would be the Devil of Hell. You must make your choice.”

No in-between path can be taken to be truly Christian. In which case, the multitudes that do take just such paths, while still occasionally taking a pew, and the similar numbers of those who profess no formal religion, but maintain a hazy conviction that there must be some originator of the universe, may make up the millions of what could be thought of as Britain’s hidden religion – a deist faith that the world has forgotten.