In recent years, the term "Celtic Church" has generally been replaced by "Insular Church" to reflect the fact that Christianity didn't survive just in Ireland and Scotland, but also in areas of Wales and England instead. Patrick Wormald in particular condemns the use of "Celtic Church" on the basis that it retroactively creates a false dichotomy between rival "Celtic" and "Roman" Catholicisms when, in reality, the Insular Church represents more of a cluster of surviving churches largely distinguished through doctrinal rather than theological idiosyncrasies. Indeed, there is no evidence of a unifying 'Celtic' theology, or even continuity of doctrine between churches in Wales and Ireland, despite the most fervent hopes of 19th Century Romantic 'historians'.

The 'departure point' for the Insular Church, if you will, was the arrival of the Pagan English in the early-mid 5th century, and the resulting increasing alienation of the Church in the geographical periphery of the British Isles from continental Europe. The separation wasn't sudden and entire; we know for example that St Germanus of Auxerre visited England in the 430s and stayed in St Albans, where he preached against Pelagian heresy that had supposedly become popular across Britannia. Pelagianism was a British heterodoxy, the terms of which remain incredibly vague, but which broadly speaking held that a just and benevolent God wouldn't countenance original sin, and therefore free will allowed a person to live perfectly without the need for divine intervention. Obviously this was a major departure from the prevaling contemporary doctrine of Augustine of Hippo, and Pelagianism was condemned at the 418 Council of Carthage, but remained prevalent in its native Britain. While there's no evidence that Pelagianism was a dominant theological force behind the Insular Church, it is possible that an apparent tradition of unorthodoxy in the church in Britain facilitated a level of syncretism with Brythonic and Gaelic cultures, and allowed the Church to survive despite its geographical separation from Rome.

The Roman Church returned to England following the Augustinian mission to Canterbury of 597, and soon favour in Kent, East Anglia and Northumbria. The "clash of Churches" that resulted in the 7th century was less a conflict of rivals, and more a process of realignment between the syncretic traditions and practices of the Insular Churches and the mainstream Roman tradition. Most of the differences between the two were doctrinal rather than theological, often as a result of the inability of the British churches to remain up to date with continental changes in doctrine. Insular beliefs around penance, for example, revolved around a more transactional form of confession and penance rather than the contemporary Roman belief in more personal contrition; interestingly in this case the Insular traditions of penitentials and indulgences had a strong influence on the development of Roman penitential doctrine. Other differences came down to things like the dating of Easter and certain monastic traditions and trappings.

The influence of the Insular Church declined commensurately with the growth of mainstream Roman Catholic English political power in the British Isles. In particular, the 664 Synod of Whitby saw Oswui of Northumbria align his kingdom - at that time the dominant political power in England - firmly with Roman traditions and doctrines. As a result of Northumbrian political influence, Roman Catholicism saw increasing acceptance in areas that had been adherents to Insular doctrine. The traditions of the Insular Church don't vanish overnight, of course, but they do decline from this point, especially in the 11th Century in Scotland - where Queen Margaret influences her husband Malcolm III to bring the Scottish church more firmly in line with the English, and thus Roman, Church - and the 12th Century in Ireland, where following the Cambro-Norman invasion of 1169-71, the Irish Church pledges to follow the English Church at the 1171 Synod of Cashell.