The condition we diagnose as softening of the brain may occur when we stop asking it to do more than it can. My father, who had a great respect for intelligence, was a Yorkshire miner all his life, although he left the pit at 14. His childhood ambition was to follow his father and grandfather to the coalface as a "ripper": paid extra wages for hacking away at the seam with a short-handled pick. He served his apprenticeship snatching out the shale from the coal as it passed on a conveyer belt in the colliery yard; but on the day he asked for promotion to work underground, his dad refused, saying he would be the last of the France family to spend his working life shut away from the light. My father left the pit and was apprenticed to a butcher, the trade he followed for the rest of his life.
He saw the miners as storm troopers in the class conflict and identified with them in their struggle against the injustices of a capitalist society. He scorned the capitalist press. All journalists, he said, were in the pay of the bosses.
Only education could help us distinguish truth from propaganda and my father's commitment to truth was obsessive. He bought us children's encyclopaedias and not fairy tales for Christmas. And he rejected Christianity because "it told lies to keep the poor quiet". We sang, at the Methodist Sunday school, that the rich man in his castle and the poor man at the gate were God's idea. So God was on the side of the Tories. The only comfort we had was that He wouldn't let them into the kingdom of heaven.
I joined the C of E when offered sixpence a week to sing in the choir there. The services confirmed my view of the middle classes as a mendacious lot: every Sunday they stood up and told each other that they were miserable sinners with no health in them. Not one of them believed it to be true.
As the scepticism of a grammar school education began to take root, I saw Christianity as the religion of losers. What was on offer from the pale Galilean was a grey lifetime of deprivation against the doubtful possibility of a hereafter in the company of churchgoers. I could see how it might appeal to the sick or the elderly who made up the regular church congregations, but as a healthy 16 year old, I preferred a bird in the hand.
I went up to Oxford five years before Look Back in Anger - that is, five years before it suddenly became trendy to be working class. So I missed out on celebrity status but was treated amiably enough for an unsociable northerner with a chip.
For me, it was important not to stand out and I was carried along by the heady iconoclasm of undergraduate life. Nobody talked about Christianity; people were rather keen on rational intelligence.
I joined the colonial service and spent years in the Fiji Islands. Here I met and became familiar with a handful of Roman Catholic priests - men with university degrees, who managed to believe that God made the world and that they had mystic powers to work transformations of bread and wine with their hands. They brought home to me the enduring mystery of religion: not that it might be true, but that so many otherwise normal and intelligent people can bring themselves to accept it.
As I tried to solve this mystery, I plunged into scholastic theology - the nights are long in the tropics and there's little to do but play bridge and get drunk. I was fuelled by its claim to prove the existence of God "by the natural light of human reason". But the summae did not bring the certainties they promised. The argument that the human mind has to postulate an Unmoved Mover or First Cause because it cannot tolerate an infinite regression didn't work in my case, since I was as uncomfortable with the postulate as with the infinite regression.
Back in England I worked as a reporter on the BBC Everyman series which, although part of the Religious Department, was staffed mainly by agnostics. (Given the BBC's passion for impartiality, agnosticism was the only position from which it was deemed possible to be fair to all faiths.) For 12 years I made films across the spectrum of religious beliefs without detecting a rationally acceptable foundation of truth in any of them.
When we first made a documentary film about Russian Orthodoxy I was astonished to find a group of Christians with such a relish for life. In England, we associate Christianity with moderation and propriety. English Christians tend to guard against the sins of the flesh by being embarrassed about their appetites. But these Russian Christians ate and drank hugely, would sing all night given half a chance, and were passionate about music, mushrooms, high-speed driving and vodka. When I asked if their Christianity was coloured by their national character, they protested that, on the contrary, a zest for life was the essence of the Christian faith. Since God had become incarnate, the material world was transformed and fleshly things were to be enjoyed as part of divine worship. I didn't entirely follow the theology, but I approved of its effects.
When, some years later, I first visited Greece, I was struck by the absence of that commitment to moderation and proportion my Classics teachers had led me to expect. Greeks we met not only sang, danced and ate prodigiously, they refused to go to bed. The common acculturating factor between the vast continental masses of Russia and this small corner of the eastern Mediterranean was Orthodox Christianity. Its effects were clearly the opposite of the western version. It was worth investigating.
The historical credentials of Orthodoxy were impressive: Christianity had its roots in the east: the Gospels were written in Greek; the councils were held in Greek; the dogmas of the church were first formulated in Greek; the most authoritative fathers of the church taught and wrote in Greek. The effects of Orthodoxy on the full life were encouraging. But there remained the question of its truth. Just because so many people over the years have believed in the existence of a Christian God is no guarantee that He is there.
In the absence of a direct revelation, I had to search for the objective truth of Christianity using the only instrument available to me: unenlightened reason. Here the Orthodox theologians helped. First, by insisting, in contrast with the Aristotelian rationality of western scholastics, that the fundamental truths of Christianity are mysteries that we can never fully understand; and second, by pointing out that by insisting on objectivity, I was failing to understand that realities exist that are neither objective nor subjective.
I had to admit that the most significant and enduring influences in my own life - my love for my wife, the power of great music and the beauties of the natural world - were beyond the power of reason to explain. To rely on the intelligence to discover all truth is, I finally realised, unintelligent.
I was baptised on Patmos at the age of 57 because I had seen and admired the effects of Orthodox Christianity on people, been impressed by its claims to be the custodian of the original Christian revelation and accepted that I could come to understand it only through the experience of membership. My rational objections as to its truth had been removed by rational enquiry.
Through faith, my area of ignorance has expanded. I had assumed that people accepted religion because they could not tolerate uncertainty; that faith brought the psychological relief of providing answers to the great questions.
Today I know far fewer things than I did as an agnostic. The materialist certainties - that life emerged from a primeval sludge, that after death there is nothing, that humanity has no purpose but will one day cease to exist - have gone. Christianity, I am discovering, is not a comforting illusion that provides answers, but the validation of mystery. And I can now accept that the most important things in my life are those that I do not now, and never will, fully understand.