Cross under the crescent

From the Holy Mountain by William Dalrymple, HarperCollins, pounds 18

When the great Byzantine traveller-monk John Moschos set out from the scorched wasteland of the Judaean hills in AD 578, his aim was to collect the wisdom of the sages and mystics of the Byzantine East before their fragile world finally disappeared. Now, almost a millennium and a half later, the travel writer William Dalrymple has spent six months following in Moschos' footsteps, starting on the Holy Mountain of Mount Athos and working his way through to the Coptic monasteries in the badlands of Upper Egypt. His aim was to witness and record the ebbing twilight of Middle Eastern Christianity.

Dalrymple travels by train, ferry, lorry and night bus, staying mainly in monasteries. He observes the flickering iconography of crumbling Byzantine churches lit by 100 candles, picks his way along the medieval lanes of bazaar towns on the Silk Route and listens to liturgies unchanged for 1,000 years. The prose rings with the sound of toponyms guaranteed to induce a Proustian rush: the Tigris, the Euphrates, Aleppo, Antioch, Alexandria, Samaria, Nazareth.

In southern Turkey the atmosphere is so tense that local newspapers can only be bought from police stations, while in the silence of a stygian crypt, Dalrymple gazes on the mummified face of a long-dead patriarch, "the left ear flat and shrivelled like an old buckle". His book is a rich stew of history and travel narrative spiced with anecdote, opinion and the bons mots of obliging locals: a hotel receptionist twirls his moustache before announcing: "Kurdistan is like a cucumber. Today in your hand; tomorrow up your arse."

Travelling alongside Dalrymple, it is clear that the position of Eastern Christians - the last surviving bridge between Islam and Western Christianity - has become increasingly untenable as a result of the hostility of the Islamic establishment. What he repeatedly points out, though, is Islam's considerable debt to the early Christian world and the degree to which it has faithfully preserved elements of the early Christian heritage long forgotten by ourselves.

Conversely, he writes that the influence of Eastern asceticism on the medieval west was "as clear and unstoppable a one-way traffic as the reverse cultural invasion of fast food and satellite television is today". In a key paragraph, he notes that while the West often views Islam as a civilisationhostile to Christianity, in Christianity's Eastern homelands you realise how closely the two religions are linked.

Moving effortlessly between centuries and millennia, Dalrymple shows how history has shaped the present. How the diversity fostered under the Ottoman Empire, for example, yielded in Ataturk's new nation to the cultural and religious homogony that has been the undoing of the Kurds. It is apparent, by the end, that the onslaught on the Christian East witnessed by Moschos was the first stage in a process whose denouement is now taking place: Christianity's terminal decline in the land of its birth.

Dalrymple uses the diary format to render this robust pottage digestible and in contentious territory he sensibly relies on great swathes of direct speech. He is an enviably accomplished stylist who has matured a good deal as a writer: the two silly girls skittering through the pages of In Xanadu and the irritating domestic trivia of City of Djinns have vanished. In three books he has metamorphosed from coltish student to dignified man of letters.

This is a book about much more than the ever-accelerating exodus of the last Christians from the Middle East. It is an attempt to penetrate the Byzantine mind. Like many travel writers, Dalrymple is at his best when he stops moving and starts thinking. As the guidebook writers colonise the last wildernesses, the future of travel literature lies in the hands of gifted authors like Dalrymple, who deploy geography to shine their torches into the shadowy hinterland of the human story - the most foreign territory of all.