Jeremias's monastery, Iveron, like 19 of the 20 monasteries on Athos, is on Byzantine time; 12 o'clock is sunset. His day starts at 3am by a normal watch when the talento is sounded - a long piece of wood struck by a mallet as a signal to the monks to wake up. The world they awake to is as close to medieval as one is likely to find anywhere.
After the talento come five hours of worship, followed by a meal that counts as lunch, even though it is eaten at 8am. The staple diet is fish, which comes cold, with wine, cheese and vegetables. Digestive problems are eased by a reading from a holy work. There is no conversation at meals.
His day is divided into worship, rest and work. Jobs are assigned by the abbot once a year on 1 January, when monks can change occupation. It is not all medieval though. Alongside the 11th-century occupations - icon painting, for example - are computer programming, dentistry and medicine.
Jeremias is the gatekeeper. The number of visitors is limited, but once they have obtained a pass from the Holy Community, the governing council of the semi-autonomous monastic state, they are entitled to free board and lodging.
No women are allowed on Athos. Even female animals are excluded. The legend goes that the Virgin Mary saw Athos's beauty and claimed it for her own garden. She was therefore the only woman allowed there. Another account says the early monks proved too vulnerable to the charms of local shepherdesses, so the authorities banned women to curb monastic lust.
Father Jeremias is part of a new wave of monks to come to Athos over the past 20 years. Until 1970, the numbers were declining. With an ageing population increasingly less able to keep its monasteries going, it looked as if the Athonite tradition would die out. Most monks at that time were peasants, attracted in part by the security of monasticism.
The monks who came to Athos in the Seventies and Eighties were better educated and more earnest. Most are theology graduates and still under 40. Their agenda has a fundamentalist flavour. They want to return to the pure spirit of Orthodoxy of the early church. They oppose any compromise with Catholicism.
Father Paul, aged 30 and with a degree from Avignon University, France, is an example of the mix of modern and medieval. An expert in art restoration, he drives a Dodge four-wheel drive like a rally driver on the dirt roads of Athos. That there are no road accidents, he believes, is proof of the existence of miracles. He has a mobile phone for the mainland, which will soon work on Athos, too. He sees Athos as a living museum. "I feel a monk is the protector and conservator of the objects there." He aims to re-create the purity of the Orthodox tradition in the artworks he restores.
There is debate on Athos about how far modern technology compromises its traditions. There may be a desire to retreat from the world, but this is no more possible to achieve completely in the 20th century than it was in the 11th.
That, however, may simply allow Orthodoxy a new lease of life. Father Artemios of the monastery of Grigoriou explains: "The truth must be kept as it is. If you change one dot or comma, all the faith, the civilisation, the cosmo-theory will be changed."
The author reported from Mount Athos for 'Europhile' on BBC Radio 4.