Our principal and most reliable information about this religion, is derived from Caesar. His account of the learning of its druids, of their knowledge of astronomy, physical science, mechanics, arithmetic, and medicine, however highly coloured, is amply corroborated by the casual statements of other authors. He expressly states that they used the Greek character in their writings, and mentions tables found in the camp of the Helvetii written in these characters, containing an account of all the men capable of bearing arms.
It is probable that Irish druidical rites manifested themselves principally in Sun-worship. The name of Bel, still retained in the Celtic Beltinne, indicates its Phoenician origin; Baal being the name under which they adored that luminary. It is also remarkable that Grian, which signifies the sun in Irish, resembles an epithet of Apollo given by Virgil, who sometimes styles him Grynaeus. St. Patrick also confirms this conjecture, by condemning Sun-worship in his Confession, when he says: “All those who adore it shall descend into misery and punishment.” If the well-known passage of Diodorus Siculus may be referred to Ireland, it affords another confirmation. Indeed, it appears difficult to conceive how any other place but Ireland could be intended by the “island in the ocean over against Gaul, to the north, and not inferior in size to Sicily, the soil of which is so fruitful that they mow there twice in the year." In this most remarkable passage, he mentions the skill of their harpers, their sacred groves and singular temple of round form, their attachment to the Greeks by a singular affection from old times, and their tradition of having been visited by the Greeks, who left offerings which were noted in Greek letters.
Toland and Carte assume that this passage refers to the Hebrides, Rowlands applies it to the island of Anglesea; but these conjectures are not worth regarding. We can scarcely imagine an unprejudiced person deciding against Ireland; but where prejudice exists, no amount of proof will satisfy. It has been suggested that the Irish pagan priests were not druids properly so called, but magi; and that the Irish word which is taken to mean druid, is only used to denote persons specially gifted with wisdom. Druidism probably sprung from magism, which was a purer kind of worship, though it would be difficult now to define the precise limits which separated these forms of paganism. If the original pagan religion of ancient Erinn was magism, introduced by its Phoenician colonizers, it is probable that it had gradually degenerated to the comparatively grosser rites of the druid before the advent of St. Patrick. His destruction of the idols at Magh Slecht is unquestionable evidence that idol worship was then practised, though probably in a very limited degree.