It was 1,400 years ago tomorrow...

...that St Columba, the early Christian pioneer to Britain, died. One of his first conquests was Mull.

If You happened to be passing Archie MacCallum's garage at Craignure on Mull at 2pm on Friday 28 March, you would have seen something odd: five youths wearing jeans and sandals, making their way through rough heather with heavy crosses on their backs, being overtaken by trucks and motorcycles from the ferry.

"On their way to the holy isle," says Archie, jerking his thumb in the direction of a fog cloud, before turning back to his pump. "We get all sorts up here."

With 600,000 visitors per annum, 40 per cent of whom are taking day-trips to the holy island of Iona at the western tip of Mull, visitors like these cross-bearers (who turned out to be Christians from Alberta, Canada) hardly raise an eyebrow any more.

Ever since AD563, when St Columba sailed across the Irish Sea in a reed coracle to found a Christian settlement here, the island has been a magnet for spiritual seekers of all sorts. Celtic anchorites, Augustinian nuns, Benedictine monks, ecumenical Christians and ardent New Agers, there is no shortage of religious variety in this three-mile strip of granite and sand.

And though modern arrangements are different - ferries, coaches and B&Bs instead of shakedown hostelries and barefoot penance on the Street of the Dead - fundamentally it's the same. You are still contemplating mass movements of people for mysterious reasons.

Little is known for certain about Columba, but by any account he was a remarkable man. Born into a royal Irish family, he was a scholar and soldier priest. He founded many monasteries in Ireland but was one day accused by the king of illegally copying a book of Psalms. Columba refused restitution of his ill-gotten treasure - it seems that he loved books - at which point there was a pitched battle. Columba and his followers won, but at great cost of life.

Repenting his bloodshed, Columba went into exile with 12 other monks. He sailed across the Irish Sea in search of a piece of land from which his homeland would not be visible.

He found it in the pink granite rocks, sloping machair land (cliff-top grassland), shallow waters and brilliant white sandy coves of Iona. After gaining permission from the then king of Dalriada, he founded a settlement there from which his monks went forth and Christianised much of Scotland and northern England and left their traces (great illuminated manuscripts and Celtic stone crosses) as far away as the Danube and the Alps.

Little remains of the original community. You can see the hillock where Columba built his wood and wattle cell; a shallow hollow in the stone where he is supposed to have sat; and stone supports where he placed his wooden work board. And it is true that the doves (a dovecote was established here in the middle ages) have a way of congregating on Torr an Aba (the Hill of the Abbot) as if in honour of its namesake. Beyond this there are the relics of a 12th-century nunnery, Reilig Odhrain, the ancient burial ground of the Scottish kings, and the restored abbey, currently used by the ecumenical Iona Community.

"That man is little to be envied," wrote Samuel Johnson in 1777, "whose patriotism would not gain force upon the plain of Marathon or whose piety would not grow warmer among the ruins of Iona."

And it is true that the island remains remarkably untouched. Looking west with nothing between you and America but some cormorants clipping across the violet horizon, you feel a living link with human aspiration at the very least, if not something more substantial.

Nor is this pull felt only on Iona. Just across the water, the tiny isle of Erraid has become home to a dynamic and vibrant New Age community.

Erraid was made famous by Robert Louis Stevenson's Kidnapped. It was here that Davie Balfour was shipwrecked after being cheated of his heritage by his wicked Uncle Ebenezer. He spends three days and three nights on the island living off limpets and buckies (periwinkles) which cause him to vomit violently after each meal, while running frantically from lookout rock to shore at the sight of a passing ship, unaware that Erraid is a tidal islet, twice a day accessible across a narrow sandbar at low tide.

Stevenson's father was Keeper of the Northern Lights and spent several years on Erraid quarrying granite for his lighthouses. It is not hard to imagine that the young Stevenson would grow up with a healthy respect for the sea, about which he writes so brilliantly.

The same respect characterises Erraid's new tenants, a split-off cell of the New Age Findhorn Community in Morayshire, who have come to this lonely island to practise the pioneering simplicity of the life of their founders.

"They believe that tatties grow by praying to them, don't they?" said the local policeman when I asked him about Erraid.

A simplification, yes, but not wholly inaccurate. Findhorn members talk of attunement with nature through getting in touch with her "devas" and "elementals". And the "Findhorn Angel", which produced such startling results on the Morayshire coast, with outsize cabbages the like of which the locals had never seen, seems to be at work here too. Notwithstanding the barren peaty soil, the vegetable gardens are magnificent.

If you can escape the magnet of mysticism in the Ross of Mull, do spend time on the rest of the island. With its magnificent mountain range (Ben More, in the centre of the island, is an extinct volcano) its glacier- scored valleys, its rivers, cataracts and beaches, and wild oak and hazel woods, it has some of the most varied and lovely scenery in the Hebrides.

Boswell and Johnson found it hard going. "This was a day of inconvenience," says Johnson of traversing Mull, "for the country is very rough, and my horse was but little." Even today the narrow roads turn driving into a kind of medieval jousting, but there are occasional restaurants and B&Bs as you go. There are also beautiful walks: along the River Lussa; on Calgary bay to the west (during the clearances many of the big emigrant ships set off for the New World from this coast); around Tobermory; and along the magnificent heights of Ben More itself. If you are lucky, you will see all manner of wildlife on these walks, from otters and sea eagles to peregrines, plovers and porpoises.

As far as historical sights go, one of the most impressive is Duart Castle, home of the Macleans. The Macleans were one of the most powerful clans in the Western Isles and built stout fortresses on promontories up the coast. Duart, a 13th-century tower with dungeons, ramparts and six-foot- deep walls, was put to the torch in punishment for Maclean support of Bonnie Prince Charlie in '45.

But when the 26th clan chief, Sir Fitzroy Maclean, returning from fighting in the Crimea, saw its silhouetted ruins on the cliff, he swore he would return and restore it to its former grandeur. This he did, spending his life on the job, and giving up the ghost only when it was finished, before being laid to rest with his forefathers in the tiny windy upland cemetery on the cliffs.

One journey that has to be made is that to Staffa. It was from this trip that Mendelssohn rushed home to write the Hebridean Overture. And there is no escaping the awe-inspiring grandeur of these cliffs with their vertical hexagonal basalt columns and spouting surge.

If you look carefully out to sea, you may even see the darkened wave or silver plume of an exhaling whale.

Sea Life Surveys of Dervaig runs trips to watch the minke whale which is thought to breed around these shores after wintering in Norway. Minkes (the largest of the baleen whales at an average of 10m) are extremely curious creatures and will come right up to a boat whose engine is cut to investigate its contents. "They will watch you just as much as you watch them," says the director of Sea Life Surveys. "In fact, they will roll over on their side or go belly up, just to get a better view of you. It's an amazing experience: people either burst into tears or break out singing."

MULL FACT FILE

Tourist Offices

Oban (01631 563 122)

Mull (01688 302 182)

Ferry

Caledonian MacBrayne (01688 302017)

Hotels

A number of comfortable hotels with great views in Tobermory (Tobermory Hotel, B&B per person per night from pounds 39 (01688 302091); Mishnish Hotel, which holds Mull's annual Traditional Music Festival (01688 302009); or the grand Victorian Western Isles Hotel (01688 302012).

B&B

Many romantic places (also clean and comfortable) in Tobermory, Dervaig, Salen, Craignure. Iona has hotels and b&bs, also the Bishop's House, an ecumenical hotel where guests may sample community living for as little as pounds 16.50 per night and the MacLeod Centre, part of the Iona Community, where you will be required to participate in prayers and religious services. Phone the Mull tourist office for advice and bookings of all the above.

Self-catering

Shieling Holidays, Craignure. Superb position close to the pier where the ferry comes in, overlooking the Sound of Mull, with a whole sweep of mainland mountains ahead and Duart Castle on a promontory to the right. Good value at pounds 43.50 for a four-person shieling (cottage tent) for three nights. Camping also offered (01680 812496). It is possible to camp on Iona at the discretion of the relevant crofter. Lovely beaches and temperate climate, a chance to trace Columba's footsteps. (Again, contact Mull tourist office for information).

Food

You can do a lot worse than buy yourself a pound of mussels on Oban pier and eat fish and chips all the way round the island. If you want something grander, go to Killichronan hotel, a wonderful lodge at the head of Loch na Keal. Contemplate an elegant avenue of beeches from the firelit warmth of the old red-gold drawing room. pounds 23 for five courses and worth it just for the location.

Trips

Many wonderful trips (information available from Mull tourist office), including visits to Staffa and Iona, Col and Tiree, Treshnish Isles, Eigg and Rhum. For example, Staffa and Iona in one day on the Iolaire of Iona (01681 700358) costs pounds 10 for adults, pounds 5 for children. Whale-watching tours run by Sea Life Surveys in Dervaig (01688 400223) cost pounds 45 for a six-hour trip. Otherwise, take advice from Mull tourist office on angling, sailing, diving, golf and jet skiing.