Most Scots learn at school that Christianity arrived on Iona with St Columba in 563 AD. The truth, which historians have long known and the rest of us are still catching up on, is more complicated. Whithorn precedes Iona by 150 years as a birthplace of Scottish Christianity. In 397 AD, while Roman legions still occupied Britain - the legions would not leave for another thirteen years - a missionary named Ninian established the first Christian mission north of Hadrian's Wall here, in a corner of what is now southwest Scotland but was then very much frontier country between empire and tribes. He built a small stone church, painted it white, and called it Candida Casa. It was Scotland's first Christian building. The community that grew up around it was the first Christian settlement north of the Roman wall.

What began as a single white-painted chapel grew quickly. By the early medieval period the site had become a cathedral and a monastery, and remained a centre of pilgrimage despite the chronic political instability of the region. Galloway in those centuries was a frontier crossed by everyone - Britons, Northumbrians, Norse settlers, Gaels, Scots - and Whithorn changed hands politically without losing its religious importance. The monks who trained at Candida Casa went on to become missionaries themselves, carrying the faith into Ireland and across Alba, the older Gaelic name for Scotland. Some travelled as far north as Orkney and Shetland. The monastery's reach was extraordinary for an institution at the end of a Galloway peninsula. Pottery and glass from the Mediterranean and from western France made it to Whithorn, suggesting that monks themselves arrived from France with new technologies and crafts.

The story of Saint Eogan, founder of the monastery of Ardstraw, captures the strange international flow of this period. Eogan was an Irishman of the 6th century, captured by pirates and brought to Britain as a slave. When he eventually obtained his freedom, he made his way to Candida Casa to study. He went home to Ireland and founded a monastery of his own. The journey reverses the more common pattern - Irish saints to Scotland, Columba to Iona, missionaries westward - and shows that the traffic in faith ran both ways across the North Channel. Bishop John Healy, writing in 1912, identified Candida Casa with the great seminary of Rosnat that appears in early Irish sources and called Whithorn 'one of the chief sources from which Irish monasticism was derived.' For a building in a quiet Galloway field, the influence was disproportionate.

Politically, Whithorn passed from hand to hand. The Brythonic-speaking Britons who first received the mission gave way to the Anglo-Saxon Northumbrians, and an Anglo-Saxon Diocese of Whithorn was established no later than around 730. The Northumbrians in turn gave way to the Norse, whose ships made the same crossings as their pagan ancestors. By 1100 the area was back under Celtic control and was now part of the emerging Kingdom of Scots. The bishopric of Whithorn was re-established in 1128, and a new cathedral and adjoining priory were built on the original site. Through all of this, the small white church remained the focal point - the reason there was a town here at all, the reason kings and bishops kept investing in the place. Centuries of political change, four cultural traditions, but one stubborn stone foundation.

The Reformation closed the priory, as it closed every monastic institution in Scotland. The buildings fell into disrepair through the 16th and 17th centuries and beyond, and what was once a cathedral became a ruin in a country field. Today the site is managed by Historic Environment Scotland as Whithorn Priory and Museum. Adjoining the ruins is St Ninian's Priory, built in 1822 as the parish church of Whithorn and still in use. The original Candida Casa - the small, whitewashed late-4th-century building from which everything else here grew - is gone. Excavations have found 5th and 6th-century timbers and stones beneath later layers, suggesting where it stood. The white painted plaster that gave the building its name has not survived 1,600 years of Galloway weather. The name has. So has the unbroken line of worship at the site - 1,600 years of continuous Christian presence, in a part of Scotland that most maps still treat as the back of beyond.