"Do you mean to tell me", she asked, "that you believe in incense, the Holy Spirit, communion and all that?" I tried to untangle her doctrinal confusion, but that only made things worse. It was the moment I realised that many people outside my own world viewed a Catholic education as eccentric.
In many ways they were right; it is difficult to imagine a worse preparation for growing to adulthood in the late 1970s than five years at a boarding school run by Benedictine monks. I emerged at odds with the spirit of the Thatcherite decade in which I would soon have to make my way.
It was a defiantly non-utilitarian education, dedicated to the search for what was true and not necessarily what was useful. The life of the school was organised around an ideal, and while exams mattered, they took second place to the teaching of the Christian message.
It gave us the illusion that to be a Catholic was as natural as breathing; when we emerged into the outside world it came as a shock to realise that many people saw us as a minority sect within a rapidly diminishing Christian population.
And it was imbued with a strong sense of a historical inheritance of doubtful relevance to modern Britain. Benedictinism is a far older institution than the nation state. When the order was founded 15 centuries ago the idea of England had no meaning, and that of Britain was many centuries in the future. During the Dark Ages the monks more or less kept Western civilisation alive, preserving classical culture in the great monastic libraries and husbanding the seeds of the Renaissance during Europe's long winter.
We were encouraged to see ourselves as part of that tradition, and the world of ancient Rome sometimes seemed closer than that of the late 20th century. Our teachers cared passionately about causes that now seem stunningly inconsequential - the head of classics was the world's leading expert on Ciceronian prose rhythms and taught us to construct our translations to echo the cadences of Cicero's orations.
None of this was very much help when it came to finding a job, indeed the mixture of Christian scruples and a residual other-worldliness mitigated against ambition.
The Benedictines are very good at what they do, and their influence proved impossible to shake off, I have been humping their inconvenient moral and historical baggage around with me ever since.
But in the past 18 months everything I learnt a quarter of a century ago has been brought alive.
I have been making a television series about the Roman Catholic Church in the modern world, visiting a dozen countries on four continents, and talking to scores of Catholics at every level.
In Eastern Europe I met Catholics like Lech Walesa who had made the Catholic ideal a central weapon in the battle against communism. For the former leader of Solidarity, the first free trade union in the old Soviet Empire, religion was anything but unworldly; it had been at the heart of one of this century's greatest revolutions.
In Latin America I found stories of heroic martyrdom by priests and bishops like Oscar Romero, the Archbishop of San Salvador who was assassinated while saying mass. The Church was at the forefront of the struggle against oppression in the region in the 1970s and 80s, and here, too, the Catholic ideal made a concrete difference in the daily lives of millions of people.
My education was privileged in many ways, but I now especially value it because it was different. The experience of the past year and a half has brought home to me the value of an education that goes beyond the strictly academic. A school which expresses an ideal - and it need not be Christian - can offer children a dimension that will enrich them for the rest of their lives.
And, although I can no longer remember how to recreate Ciceronian prose rhythms in translation - or very much Latin come to that - I have come to understand the point of that very strong link with the past which the Benedictines sought to forge in our minds.
As I travelled the Catholic world, I realised that the heritage I share with my co-religionists in Asia and Africa gives us membership privileges in the world's biggest club; the Church is around a billion strong, and in many areas is growing fast.
Only a tiny proportion of British children will have the kind of private, Catholic education I enjoyed. But my experience does carry alesson with a wider application, and it is one easily forgotten in the relentless pursuit of higher academic standards and better exam grades. A sense of the past is essential to any education, because it is a step towards understanding our common humanity.
Ed Stourton presents BBC1's One O'Clock News. His series, Absolute Truth,continues on BBC2 on Sunday at 8pm