Druids, a sacred order of learned men under a chief called the Archdruid, among the ancient Celtic nations, particularly of Gaul and Britain, who, from their knowledge of the arts and sciences of the day, were the ministers of religion and justice, as well as the teachers of youth to the whole community, and exercised an absolute control over the unlearned people whom they governed; they worshipped in oak groves, and the oak tree and the mistletoe were sacred to them; the heavenly bodies appear to have been also objects of their worship, and they appear to have believed in the immortality and transmigration of the soul; but they committed nothing to writing, and for our knowledge of them we have to depend on the reports of outsiders.

DRUIDS, n. Priests and ministers of an ancient Celtic religion which
did not disdain to employ the humble allurement of human sacrifice.
Very little is now known about the Druids and their faith. Pliny says
their religion, originating in Britain, spread eastward as far as
Persia. Caesar says those who desired to study its mysteries went to
Britain. Caesar himself went to Britain, but does not appear to have
obtained any high preferment in the Druidical Church, although his
talent for human sacrifice was considerable.
Druids performed their religious rites in groves, and knew nothing
of church mortgages and the season-ticket system of pew rents. They
were, in short, heathens and -- as they were once complacently
catalogued by a distinguished prelate of the Church of England --
Dissenters.