What types of evidence provide information about Celtic religious beliefs?

Information regarding Celtic religious beliefs is primarily derived from two main categories of evidence, both of which present unique challenges for interpretation.1

Primary Sources of Evidence

• Sculptural Monuments and Inscriptions: These artifacts, found in continental Europe and Roman Britain, date largely to the Roman period. They often reflect a syncretism between Celtic and Roman deities, making it difficult to isolate pre-Roman traditions without the aid of a preserved mythological literature.1
• Insular Celtic Literatures: These texts were recorded in Ireland and Wales during the medieval period. Because these traditions were written down centuries later by monastic scholars in a Christianized context, the literature reflects the influence of Christian learning and the specific selectivity of the redactors.1

Additional Contextual Evidence

Beyond these primary sources, historians rely on the observations of Classical commentators, such as Poseidonius, who recorded their own or others' accounts of Celtic culture and religious practices. Furthermore, archaeological discoveries—such as forest sanctuaries and, later, Gallo-Roman temples—provide physical evidence of ritual spaces. Despite the chronological and geographical distance between these various sources, scholars have noted a remarkable number of points of agreement between the continental archaeological evidence and the later insular literary traditions.12