The Druids were the priests of the ancient Celts.
The Druids among the Celts . . . devoted themselves to philosophic pursuits. Hippolytus (c. 225, W), 5.9
The Celtic Druids investigated to the utmost the Pythagorean philosophy. . . . The Celts esteem the Druids as prophets and seers, on account of their foretelling to them certain events, from calculations and numbers of the Pythagorean art. . . . The Druids resort to magical rites as well. Hippolytus (c. 225, W), 5.22