Post-Roman Scotland

In the wake of the Roman withdrawal Scotland's population comprised two main groups:

1. the Picts, a people of uncertain origin (but possibly a Brythonic Celtic group) who occupied most of Scotland north of the Firth of Clyde and the Firth of Forth: the area known as "Pictavia"
2. the Britons formed a Roman-influenced Brythonic Celtic culture in the south, with the kingdom of Y Strad Glud (Strathclyde) from the Firth of Clyde southwards, Rheged in Cumbria, Selgovae in the central Borders area and the Votadini or Gododdin from the Firth of Forth down to the Tweed

Invasions brought three more groups, though the extent to which they replaced native populations is unknown

1. the Old Irish-speaking Scotti (Irish) or more specifically, the Dal Riatans, arrived from Ireland from the late 5th century onwards, taking possession of the Western Isles and the west coast in the Kingdom of Dalriada.
2. the Anglo-Saxons expanding from Bernicia and the continent. Notably seizing Gododdin in the 7th Century. A legacy of this influence is the vernacular Scots language, a Germanic language similar to, but distinct from, English. The language was initially termed Inglis but this terminology became unpalatable after Anglo-Norman had been eclipsed by the English language within England from the late 14th century onwards. Gaelic, which had earlier been referred to as "Scottis" (pronounced the same way as Scots), was increasingly referred to instead as Erse, the word used for Irish. This terminology has now fallen out of use within Scottish Standard English, however, and Gaelic is now normally used instead.
3. in the aftermath of the 795 Viking raid on Iona, the Norse Jarls of Orkney took hold of the Western Isles, Caithness and Sutherland, while Norse settlers mixed with the inhabitants of Galloway to become the Gallgaels.

The British Saint Ninian conducted the first Christian mission in Scotland. From his base, the Candida Casa (present-day Whithorn) on the Solway Firth, he spread the faith in the south and east of Scotland and in the north of England. However, according to the writings of Saint Patrick and Saint Columba, the Picts appear to have renounced Christianity in the century between Ninian's death (432) and the arrival of Saint Columba in 563. The reason is not known. The Gaels re-introduced Christianity into Pictish Scotland, gradually pushing out worship of the older Celtic gods. The most famous evangelist of that period, Saint Columba, came to Scotland in 563 and settled on the island of Iona. Some consider his (possibly apocryphal) conversion of the Pictish king Bridei a key event in the Christianisation of Scotland.