Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Religious Union of the Nation - Druids
The universality and the strength of the Celtic national feeling would be inexplicable but for the circumstance that, amidst the greatest political disruption, the Celtic nation had for long been centralized in respect of religion and even of theology. The Celtic priesthood or, to use the native name, the corporation of the Druids, certainly embraced the British islands and all Gaul, and perhaps also other Celtic countries, in a common religious- national bond.
It possessed a special head elected by the priests themselves; special schools, in which its very comprehensive tradition was transmitted; special privileges, particularly exemption from taxation and military service, which every clan respected; annual councils, which were held near Chartres at the "centre of the Celtic earth"; and above all, a believing people, who in painful piety and blind obedience to their priests seem to have been nowise inferior to the Irish of modern times.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=32