foundation · 400
Bede reports that the southern Picts were converted long before the time of Columba by Nynia, a British bishop regularly instructed at Rome, whose episcopal see was named for Saint Martin and commonly called the White House because he built a church of stone there, which was unusual among the Britons. That is very nearly everything. Bede was writing three centuries later and clearly had a source, but not one that survives, and the archaeology at Whithorn shows an early Christian site without settling who founded it. Everything else — the dates, the miracles, the details of the mission — comes from a twelfth-century life by Ailred of Rievaulx, which is a long way after. This dataset grades the event traditional and keeps Bede's three claims and no more: a Briton, trained at Rome, a stone church, and the southern Picts. Whithorn itself is not in the fetched entity set, so the event carries Roman Britain rather than a guessed identifier — an honest coarseness rather than a false precision.
How much can rest on three sentences?
A national origin story, in this case.