As far as archeologists can determine, the Celtic peoples originated near the Danube River and spread east, south and west from there. Today, the only identifiable Celtic populations are found in France (Brittany) and the British Isles (Ireland, Scotland and Wales). Two thousand years ago they were all over southern Europe.
They lived along the Po River in northern Italy, in Switzerland, Belgium, France, Spain, all over the British Isles, into Bosnia and as far as Asia Minor (present day Turkey). The Greek form of Celts is Galatai. In France they were known as Gauls, in Asia Minor they were Galatians.
The Apostle Paul brought the gospel to the Galatians. Believers from there took it to the Gauls in southern France and from there it spread into the British Isles. It may have been Celtic missionaries from Scotland that carried the gospel to northern Italy, Bohemia and Switzerland. In time the gospel spread from the Celts to the people around them.
The Celts never organized into nation states, they were more a loose association of clans. As long as they were able to maintain their independent existence, the gospel that took root among them was of a purer form than the syncretistic gospel that was imposed in the Roman Empire after Constantine.
As Germanic peoples moved into the territories occupied by the Celts and the Roman Empire extended its reach, the Celtic peoples were absorbed into the majority culture. Nevertheless, evidence remained of their purer gospel among the faith groups known as Waldenses in the Alps, Albigenses in southern France and Bogomils in Bosnia. There is historical evidence of links between these groups, preachers from Bosnia appearing in the south of France, in Italy, Bohemia and other places.
These old evangelical brethren believed that Christians were citizens of the kingdom of God and were not to take part in governing earthly kingdoms. The Roman Catholic church accused them of being dualists, of believing that the God of the Old Testament was not the same as the God revealed in the New Testament. There is historical evidence of that belief in many of the same areas, but the faith groups named above did not hold such a belief. It was merely a handy accusation to justify using political power to persecute rivals to the Roman Catholic church and taint all evidence of the purity of their faith.
Eventually these churches appeared to have been persecuted into oblivion. Yet the faith proved to be more resilient than the persecutors. New churches sprang up in Switzerland, south Germany and the Low Countries, professing the same old faith. They came to be known as Mennonites. There is one intriguing last glimpse of the old churches in eastern Europe. In the 16th century, three men from the region of Thessalonika travelled to Germany because they had heard there were fellow believers there. They met with a Mennonite congregation, found they were united in all points of their faith and held communion together.