With regards to references to Druidism in Classical times we have, basically, short (one or two sentence) mentions in Caesar, Diodorus, Strabo, Pomponius Mela, Tacitus and Cicero - of these Cicero talks directly about a Druid, Divitiacus, that he personally knew.

Pete

God is our guide! from field, from wave, From plough, from anvil, and from loom; We come, our country's rights to save, And speak a tyrant faction's doom: We raise the watch-word liberty; We will, we will,we will be free!