IN describing the Irish and Welsh vernacular sources for Celtic religious beliefs, the Irish scholar Proinsias MacCana, in a memorable phrase, referred to the 'fertile chaos of the insular tradition'. 'Fertile chaos' perfectly characterizes the enormously rich array of partial and confusing evidence available to us from a wide variety of sources reflecting on the religious beliefs of Celtic peoples. It may fairly be said that there is more, varied, evidence for Celtic religion than for any other aspect of Celtic life. The only problem is to be able to assemble it in a systematic form which does not too greatly oversimplify the intricate texture of its detail.
The Greek and Roman texts provide a number of pertinent observations, but these are at best anecdotal, offered largely as colourful background by writers whose prime intention was to communicate other messages. The most comprehensive account is that given by Caesar in his description of Gaulish society. Though useful, its summary nature could lead to confusion if it was not balanced against other evidence. Similarly the immensely rich vernacular literature of the Insular Celts must be approached in the awareness that Celtic religion was not necessarily consistent across Europe, nor was it unchanging. The very 'Insular' nature of the sources and the fact that what comes down to us has passed through the emasculating filter of a Christian monastic perspective demand particular care when such material is used as a basis for generalization.
The archaeological record produces an equally varied mix of data. Iconography in the form of stone and wood sculptures is not particularly plentiful, but, if Celtic art in its broader sense is taken into account, as it must be, the variety of symbols and images is greatly multiplied. In addition to this we have evidence for belief systems embedded in burial practice, in a great number of votive deposits, and in religious structures such as shrines and shafts. From this huge mass of disparate evidence, sometimes distorted and usually partial, some semblance of the religious systems of the Celts can be reconstructed.