Faith and Reason: Maybe a trick, but it is still a treat: John Cornwell, the author of Strange Gods, observes the annual liquefaction of the blood of St Januarius, a miracle which, the faithful believe, preserves Naples from catastrophe.

I AM standing on the high altar of Naples Cathedral on the Feast of Saint Januarius amidst dignitaries within a roped-off section bordered by banks of gladioli. There are a dozen coquettish Neapolitan ladies fashionably dressed in large hats, an assortment of local politicos and padroni in electric-blue suits and dark glasses. One woman is fervently praying the rosary while her companion, a sinister bulge in his jacket, is scanning the stock market report in the Messaggero.

The packed congregation bursts into applause as a procession starts its progress from the sacristy to the accompaniment of steel bands. Following up the rear is the Cardinal of Naples, a humble-looking little man who looks as if he would rather be elsewhere. And here come the bearers with the blood reliquarium of Saint Januarius, the fourth-century martyr killed by bears in the amphitheatre of Pozzuoli.

As the procession reaches the altar a line of women in black closes in on the rails below: these are the formidable Zie di San Gennaro (the Aunts of Saint Januarius), who will scream out and curse the relic if it fails to do its stuff. They look in no mood to be messed with.

His Eminence, not a Southerner, begins to speak in a small tired voice, and within minutes it is clear that he has no appetite for miracles of the sort he is about to preside over. Incredibly, he is lecturing the 6,000 Neapolitans on the need to believe without seeing. And as he continues it is all too obvious that he is not carrying his congregation with him, and especially not the prelates and priests in the choir stalls - one or two of whom are yawning extravagantly and looking at their watches.

When he comes to a conclusion a gasp of anticipation rushes through the vast cathedral like a passing express train. Now to the important business. The miracle. The cardinal takes hold of the reliquarium containing the 'blood', a solid mass of black stuff which fills half the phial. The priests pray a litany, then a Credo, then another litany. The cardinal shakes the phial, but it stays stubbornly solid. The little aunts are getting restless down there. A prelate comes forward and whispers in the cardinal's ear, whereupon His Eminence begins to blow hard on the glass phial as if he were trying to breathe life into a dying fire.

Suddenly the cardinal stops blowing. He blinks; then his face is suffused with relief. Something is happening. The prelate comes forward and inspects the phial closely. He is smiling complacently. He summons a fop in scarlet sash and tails who produces an enormous white handkerchief, shakes it twice, then holds it aloft.

The cathedral shakes to the rafters with shouts and whistles. The blood of Saint Januarius has once again liquefied. The city will not, after all, disappear under Vesuvius. The city is safe for at least another year.

The cardinal is turning the reliquarium this way and that. He comes forward to the pen where the dignitaries are standing and shows the phial to each of us in turn. As he thrusts it towards my face he manages to clout me on the nose with it. The solid, dry looking lump is now shiny and viscous; the edges have turned dark brown. Fine grainy specks are suspended in muddy liquid and the mass inside the phial is swimming about like a lump of lamb's liver.

What did this English observer, unaccustomed to such Italianate excesses, feel at that moment?

The organ was surging and the bells of the cathedral were pealing. I was immensely impressed; for the original mass had seemed as dry and as solid as stone. I was thinking, 'Wow]' And yet, the prodigy failed to exert the slightest sense of moral power for me personally; at least, not at that moment.

The moment the evidence of the miracle presented itself before my eyes, I had found myself merely ruminating on how the trick had been done: beeswax? Honey? Some sort of clay that melts at low temperatures? And what if I had witnessed a manifestation of God's direct suspension of the laws of physics? Would this not equate the Godhead with temperament of a five-year-old? Would this not have less to do with God's mercy, his action in the world, than with procurable and primitive emotions of superstition, propitiation, manipulation?

Was the congregation assailed by any such scruples? The crowd was ecstatic, roaring, threatening to take the roof of the cathedral. Some serious-looking foreigners, Swiss, perhaps, were peering forward with puzzled looks, much as I had done. One of them might have been a correspondent for Nature magazine examining an experiment in physical chemistry.

The 'miracle' of Saint Januarius had been not so much a display of power, of the prodigious, as a jubilee of rebirth, resurrection, reawakening. It was as if Naples had scored the winning goal against Juventus. Except that it was the city itself, along with its ancient, its local pieties, that had somehow scored, risen above its perennial miseries, its disadvantages, its demoralisation. Christian and pre-Christian sacramentals and emblems, all seemed to blossom and ignite in the culmination of this extraordinary ritual. Seen in this light, it mattered not a jot whether the phenomenon was authentic, or, along with the local liquefaction of the Virgin's milk, and the two liquefying heads of John the Baptist, not quite all that it seemed.

But already the phial had been placed back on its stand on the side of the altar. The members of the congregation were embracing and kissing, turning their backs - so soon? - on the 'miracle', and making for the wide-open doors and the daylight reality of the city streets beyond.

As for the prelates and priests, they were charging down the nave in a motley scramble, hastening towards the aroma of doughnuts and sweet black coffee that emanated from the sacristy.