Nothing annoys me more than a magnificent Renaissance painting which carries the deadly label "school of". Why any of the great masters would let some junior copy or finish off his martyrdoms and crucifixions perplexes me, although – in an age when paintings were commissioned by popes and dukes – speed and commercial success were probably more important than artistic pride.
Indeed, it was only when I began to examine the provenance of Hizbollah "martyr" portraits in Lebanon that I discovered the same principle applied. The top painter of Hizbollah's dead – those young men invariably shot, blown up or bombed to death by Israel – is a man called Shelala.
But when I tracked his studio down in the Jnah suburb of south Beirut, I found that he would instruct a team of enthusiasts how to paint the "martyr''s face – how big his beard should be, whether the tulips should be on the right or left of his head – and let them get on with the work. He would drop by later to touch up an eye or a pair of spectacles on the dead man's face before the finished product was carted off to be strung on an electric pylon or a cemetery wall in southern Lebanon. "School of Shelala".
I have to say that Pinturicchio of Perugia – Bernardino di Betto di Biagio for Renaissance scholars – is a cut above Shelala. His 15th-century virgins and saints have that luminescent quality and perspective that most Renaissance painting exults, and the Umbrian city's exhibition of his work – along with the confusingly named Perugino of Castel della Pieve (Pietro di Cristoforo Vannucci) – is a claustrophobic but dutiful collection of religious pretension and obedience.
Yet I had forgotten the degree to which these two men – along with their "schools" and countless other minor artists across Italy – focused their attention on martyrs and anchorites, lonely old hermits who live out their days in grim contemplation of God's goodness and cruelty.
The martyrs are familiar enough. Christ's body and blood are set pieces, the red fountains always pouring from identical wounds, the feet bleeding into little piles of gore where miniature but obsessive monks can be seen staring at the stuff with unbecoming enthusiasm.
The violence of the age marries perfectly into the Shia martyrology of the imams Ali and Hussein, whose blood-boltered features dominate the posters beside the great mosques of Najaf and Kufa and Kerbala. Indeed, St Sebastian's death – all arrows puncturing white skin – is straight out of Shia martyrology.
One altarpiece I came across in Perugia this week showed a remarkably pristine version of the crucifixion, with scarcely a sign of holy wounds, until, at the bottom right-hand corner, I espied the head of St Peter with what looked like a meat cleaver in the top of his skull, from which rained the inevitable blood. His face, eyes squinting in pain, bore the expression of a man who, well, who has just been bashed over the head with a meat cleaver. A violent time, the Renaissance.
But a time of contemplation. Repeatedly, old St Jerome turns up in Pinturicchio's work. Over and over again, the ancient hermit can be seen kneeling or stooping in front of a cave amid barren mountains, shaggy-bearded – sometimes ginger, sometimes pepper and salt – staring at some distant vision.
That this was painted at a time when Ferdinand and Isabella's Spanish cartoonists were obscenely portraying another holy man – Mohamed by name – who received another message from God, only marks the thin line between devotion and hatred. Yes, late 15th-century Spanish artists far outdid the puerile cartoons of 21st-century Denmark.
But it was not just the Prophet whom St Jerome reminded me of. Who else comes to mind? Well, I can think of another man, his long beard growing whiter with age, who lives in caves and believes in visions and messages. I've even met him beside just such a cave. A night on the bare mountain must be just as bleak in Pushtunistan as it was in Umbria although the effects, as we know, can be catastrophically different.
In an age when we are still supposed to believe in the "clash of civilisations" – how anyone was taken in by Huntingdon's preposterous book is still a mystery to me – and in "faith" foundations created by equally preposterous former prime ministers, it does no harm to look at the work of my old Palestinian friend Tarif Khalidi who lives just round the corner from me in Beirut.
When he first turned up to teach at Cambridge, I pointed out of the window of his apartment and asked him if he didn't feel a bit far from home with the towers of King's College opposite his home. "But what do they remind you of, Fisky?" he asked. I thought for a moment before the obvious dawned on me. Minarets, I asked? "Exactly, Fisky!" he roared.
And so I turn to his seminal book, The Muslim Jesus, a collection of Islamic sayings and stories about one of Islam's prophets who just happens to be the subject of veneration for all Pinturicchios and for the waning number of Christians in the West. Some are clearly cribs. Muslim scholars have Jesus repeatedly raising the dead – and in one extraordinary tale, "de-resurrecting" the risen and packing them off back into the grave.
The man who tells Jesus that he has plucked out his eye after looking at a woman before being drenched in a downpour is both a Muslim prayer for rain and a reference to Matthew 18:9.
Christ's encounter with Satan in Abu Hamid al-Ghazali's story – the Devil tries to persuade Jesus to repeat "There is no god but God" but his attempt is denied on the grounds that "deceptions can lurk even beneath good" – must be a ghostly parallel of the temptation in the wilderness.
There's a story or two to take the breath away. In one, Jesus observes: "How many there are of sound body, beautiful face, and eloquent tongue who end up screaming in the tiers of hell!" And if you want a bit of good, old-fashioned misogyny, try this: Jesus said, "The greatest sin is love of the world. Women are the ropes of Satan. Wine is the key to every evil."
Now there's a Muslim prophet for you. O for the wine of Umbria!
Robert Fisk's new book, 'The Age of the Warrior: Selected Writings', is published by Fourth EstateReuse content