Celtic Christianity is a form of Christianity that was common, or held to be common, across the Celtic-speaking world during the Early Middle Ages. The term Celtic Church is deprecated by many historians as it implies a unified and identifiable entity entirely separate from that of mainstream Western Christendom. For this reason, Brown (2003) notes a preference for the term Insular Christianity. As Patrick Wormald explained, "One of the common misconceptions is that there was a Roman Church to which the Celtic Church was nationally opposed."

Some writers have described a distinct "Celtic Church" uniting the Celtic peoples and distinguishing them from adherents of the Roman Church, while others classify Celtic Christianity as a set of distinctive practices occurring in those areas. Varying scholars reject the former notion, but note that there were certain traditions and practices present in both the Irish and British churches that were not seen in the wider Christian world.

Such practices include: a distinctive system for determining the dating of Easter, a style of monastic tonsure, a unique system of penance, and the popularity of going into "exile for Christ". Additionally, there were other practices that developed in certain parts of Great Britain and Ireland that were not known to have spread beyond particular regions. The term typically denotes the regional practices among the insular churches and their associates rather than actual theological differences.

Popularized by German historian Lutz von Padberg, the term "Iroschottisch" is used to describe this supposed dichotomy between Irish-Scottish and Roman Christianity. As a whole, Celtic-speaking areas were part of Latin Christendom at a time when there was significant regional variation of liturgy and structure. But a general collective veneration of the Papacy was no less intense in Celtic-speaking areas.

Nonetheless, distinctive traditions developed and spread to both Ireland and Great Britain, especially in the 6th and 7th centuries. Some elements may have been introduced to Ireland by the Romano-British Saint Patrick, and later, others from Ireland to Great Britain through the Irish mission system of Saint Columba. However, the histories of the churches of the Irish, Welsh, Scots, Breton, Cornish, and Manx peoples diverge significantly after the 8th century. Interest in the subject has led to a series of Celtic Christian Revival movements, which have shaped popular perceptions of the Celts and their Christian religious practices.

People have conceived of "Celtic Christianity" in different ways at different times. Writings on the topic frequently say more about the time in which they originate than about the historical state of Christianity in the early medieval Celtic-speaking world, and many notions are now discredited in modern academic discourse. One particularly prominent feature ascribed to Celtic Christianity is that it is supposedly inherently distinct from – and generally opposed to – the Catholic Church. Other common claims include that Celtic Christianity denied the authority of the Pope, was less authoritarian than the Catholic Church, more spiritual, friendlier to women, more connected with nature, and more comfortable dealing with Celtic polytheism. One view, which gained substantial scholarly traction in the 19th century, was that there was a "Celtic Church", a significant organised Christian body or denomination uniting the Celtic peoples and separating them from the "Roman" church of continental Europe. An example of this appears in Toynbee's Study of History (1934–1961), which identified Celtic Christianity with an "Abortive Far Western Civilization" – the nucleus of a new society, which was prevented from taking root by the Roman Church, Vikings, and Normans. Others have been content to speak of "Celtic Christianity" as consisting of certain traditions and beliefs intrinsic to the Celts.

However, modern scholars have identified problems with all of these claims, and find the term "Celtic Christianity" problematic in and of itself. Modern scholarship roundly rejects the idea of a "Celtic Church" due to the lack of substantiating evidence. Indeed, distinct Irish and British church traditions existed, each with their own practices, and there was significant local variation even within the individual Irish and British spheres. While the Irish and British churches had some traditions in common, these were relatively few. Even these commonalities did not exist due to the "Celticity" of the regions, but due to other historical and geographical factors. Additionally, the Christians of Ireland and Britain were not "anti-Roman"; Celtic areas respected the authority of Rome and the papacy as strongly as any other region of Europe. Caitlin Corning further notes that the "Irish and British were no more pro-women, pro-environment, or even more spiritual than the rest of the Church."

Corning writes that scholars have identified three major strands of thought that have influenced the popular conceptions of Celtic Christianity: