Archaeologists trace Celtic-speaking Europe through the Hallstatt culture of the early Iron Age and the La Tene art style that followed it, spreading from the upper Danube and Rhine across Gaul, Iberia and northern Italy. Celtic war bands sacked Rome around 387 BCE, raided Delphi in 279 BCE, and settled in Anatolia as the Galatians, the same people Paul later addressed by letter. Rome absorbed Cisalpine Gaul, then Caesar conquered Gaul itself in the 50s BCE, and Britain followed under Claudius; Gaulish religion continued under Roman rule in blended forms, its gods paired with Roman ones, until Christianization.
Ireland and northern Britain, never conquered, carried Celtic languages and social forms into the Middle Ages, and Irish monks wrote down the sagas that preserve most of what is known of the mythology. Brittany was settled from Britain, keeping Breton alive on the continent. Irish, Scottish Gaelic, Welsh and Breton remain spoken today, with Cornish and Manx revived, and the modern Celtic nations built cultural revivals, from the Welsh Eisteddfod to the Gaelic League, on that inheritance.
The name itself is now argued over. Classical authors called continental peoples Keltoi but never the Britons or Irish, and Simon James has argued that the idea of the ancient islanders as Celts is an eighteenth century scholarly construction, while John T. Koch and others hold that shared languages and institutions justify the shared name. The atlas records that dispute as a live one rather than settling it.