An account of religion in modern Scotland could start anywhere but this will begin with Scotland's maritime fringes for three reasons. The first is trivial: in their very different ways Lewis, Orkney and Shetland are breathtakingly beautiful. The second is relevant: too often Scotland is reduced to Glasgow and its religious culture is reduced to sectarian conflict. The third is important: it is easier to see social processes at work in small and clearly defined areas. In 1900 the peoples of Lewis, Orkney and Shetland were similarly likely to be churchgoing. By 2000, the people of Lewis were almost three times as likely as those of the Northern Isles to attend church. And, as we will see, that divergence came before the oil industry changed Orkney and Shetland. So we have something to explain.

We also have a good chance of explaining it. The value of Lewis, Orkney and Shetland for systematic comparison is that in 1900 the three islands were similar in many respects. They were part of the same state, had the same national church and were subject to the same system of law and public administration. They were economically similar: their peoples eked out mostly thin livings in combinations of subsistence farming and fishing that were protected by the Crofters Act of 1886. There were differences in weighting – the people of Lewis and of Orkney were crofters with boats while the typical Shetlander was a fisherman with a croft – but they were more like each other than any was like the people of the lowlands.