You say a book on the popes has been at the back of your mind for 25 years. What made you finally write it?
Suddenly I didn't know what else to write about. The idea always scared me off - you know, 2,000 years, the sheer scale of it. But then I decided I would try, and enjoyed it enormously.

How did your personal meetings with popes inform the book?
Well, I've met four - Pius XII, John XXIII, Paul VI (at his coronation, as I was sent as dogsbody to the Duke of Norfolk, who was representing the Queen) and John Paul I. John Paul was the one I knew best, certainly the one I was fondest of. I'm not a religious man at all, but he seemed to radiate a marvellous sanctity. He had one of the loveliest smiles you've ever seen.

You describe yourself as an agnostic Protestant. Why immerse yourself in Catholic history?
That I don't particularly believe in God doesn't mean that I don't think popes are just as interesting as kings or emperors. In a way, they are more interesting, because over the years they've had more power. But I shall probably rub a lot of people up the wrong way. I think they will be shocked when I call the anti-Semitic Pope Pius XII contemptible. He has an appalling record, yet the Church is trying to canonise him. He ought to be roasting in Hell - if I believed in such a place.

By your account, the founding of the Church in Rome seems very vague.
There's only one sentence in the whole Bible on which the Catholic Church is based - in Matthew, when Jesus says: "Thou art Peter, and on this rock shall I build my church." People think even that is an interpretation, because it is the only time the word church is used. If it is an interpretation, the whole structure of the Catholic Church in Rome falls to the ground. The only thing to go on is that, in the 4th century, the Emperor Constantine was convinced that Peter was killed on this spot and built St Peter's Church over it.

Given its uncertain beginnings, how did the Catholic Church establish such influence?
Somehow, through all this miasma of doubt, this institution slowly scrambled to its feet. It has ultimately become the most influential religion, with two billion members, all of whom look up to one man, Benedict XVI, as God's representative on earth.

You say that Pope John Paul II, in particular, was a great populariser of the Church.
I have my reservations about John Paul II. Obviously he was instrumental in the end of the Soviet Union. For that he deserves nothing but high praise and respect. But there was absolutely no give on homosexuality or abortion. And then he did this idiotic thing, making 500 saints. No previous pope had made more than about two. Suddenly we have saints like other people have mice.

Why are you so critical of the current Pope?
I'm sure he's well-meaning, but I do think he's a terrible donkey. Within two years of his accession, he had violently antagonised the Protestants, the Muslims and the Jews, all without any good reason. He wasn't even answering questions; he just volunteered these extraordinary statements, which were received with horror.

How much damage do you think the child abuse scandal has done to the Catholic Church?
The Church has done enormous self-damage in the past five or six years. It has handled the scandal very badly. First of all, it said it was totally untrue, which is ridiculous because then you had hundreds of choirboys queuing to testify. Then it said: "Yes, but the priests have been sent somewhere else now." But they should have been defrocked instantly. John Paul I would have handled it infinitely better. He was just about to blow it open when they bumped him off.

Do you believe he was murdered?
It's such a strong case. Except I do have this one doubt: they would have poisoned his bedside glass of water, which means that someone would have had to get in there in the middle of the night. The Vatican lost its head and started contradicting itself. There was no post-mortem, no autopsy, nothing. The Vatican is the easiest place in the world to commit murder because there's no police force. It is deeply mysterious. We shall never know.

John Julius Norwich's "The Popes: a History" is published by Chatto & Windus (£25) [2]