There was once a time before 664 where Ionan Christian beliefs dominated Northumbrian Christianity, nearly upended the Gregorian mission, and rivaled papal power in the British Isles. After a few contentious years of argumentative debate over such things as the interpretation of the date of Easter, the Synod of Whitby was held leading to an agreed upon way of celebration. It also drove the Iona Abbey into isolation, surrendering the British Isles to Papal decree.
What would have happened if the Ionan interpretation had been argued more effectively, or had received more support before 664?