As the Celtic Church evangelised Northern Britain as well as parts of Western Europe a greater conflict emerged with the English Church. The Celtic missionaries had shown an unusual degree of reluctance in evangelising the Anglo-Saxons who had been arriving in England in increasing numbers since the collapse of the Roman Empire, instead working more happily and successfully among the native British tribes. The vacuum was filled by Augustine who was sent by Pope Gregory the Great to evangelise the “Angles” who appeared to him to be more like “angels”. Missionaries from Gaul were also working independently from either Rome or Ireland but the main conflict which emerged was between the mission founded by Augustine, centred on Canterbury, and the Celtic missionaries with their base at Iona. For a time the two missions coexisted uneasily but matters came to a head in 644AD at the Council of Whitby in Northumbria.

Hilda, the abbess of Whitby, was essentially, a supporter of the Celtic Church but Oswy, the king of Northumbria followed the Roman way of Canterbury. The differences between the two branches of the church eventually focused on the correct method of calculating the date of Easter. The Celtic Church followed an older eighty four year cycle in its calculations while the Roman Church, since 458AD had used a nineteen year cycle. The real issue, of course, was one of authority. Who had a mandate to change the method of calculating Easter since at the Council of Arles in 314AD it had been agreed that Easter ought to be celebrated on the same day throughout the Church. Behind this point of principle was the difficult question of the relationship between the Roman and Celtic Churches. Rome did not want to insist on absolute uniformity at this stage and the Celts did not want to be a wholly separate Church but there were evident strains.

Crucially, the matter was decided by the king rather than by church leaders; a sign that over a period of time church and “state” were becoming increasingly interwoven in both the Roman and Celtic areas of influence. The turning point in the debate came when Oswy asked Colman, the Celts main advocate, on whose authority he spoke. Colman answered, “Columba’s”. Wilfred, Canterbury's spokesman was asked the same question and replied, “St. Peter’s”. On the basis that St Peter was the keeper of the keys of the Kingdom of Heaven, Oswy chose Canterbury over Iona since he argued “I certainly want to get through into heaven.”

The Council of Whitby was a turning point in the history of the Celtic Church and in relationships between the English and Irish Churches and Peoples for a number of reasons. In the first place it marked the beginning of the decline in influence of the Celtic Church in Britain and the emergence of Canterbury as the seat of English Christianity. Inevitably, for some, the Council came to be seen as a decision of the English Church and People, over against the Celtic Church and Peoples. This feeling was highlighted by the fact that the Council of Whitby was such a mixture of theology, culture and politics. In many ways Whitby foreshadowed the complex relationships that were to dominate future Irish history: relationships not only among the Irish but between the Irish and English kings, the English Church and the papacy*. In particular, the question of the jurisdiction of Canterbury and whether or not it should extend to Ireland resurfaced a number of times in subsequent centuries, not only creating resentment in the Irish Church but, at times, leading Irish Political and Church leaders into unlikely alliances in an effort to maintain or achieve the Irish Church's independence from Canterbury.

More worryingly, within twenty years of the Council of Whitby the first partial invasion of the distinctively Celtic nature of the Irish church diminished until it eventually conformed totally

* papacy - the office of pope; the length of time a pope is in office.