The Christianization of Saxon lands was rolling right along in the early 7th century. Kent was the first to convert; then came Essex and Northumbria. Missionaries were making inroads in other of the Saxon lands as well.
But by the middle of the century, the teachings of the Christian faith had reached a crossroads. Kent and Essex and such were converted by monks and others of the Roman Church. Also alive and well and flourishing quite nicely in Britain at this time was the Celtic Church. It was Christian as well, melding elements of the old faith and the new faith. Yet, Celtic and Roman teachings differed on certain significant points as well: The calculation of Easter was the largest stumbling block. The proper cutting of the tonsure was another. (This was, of course, the shaving of the head. It might not seem like a big deal to us today, but it was a huge deal to the monks and other leaders of the church back then.)
Northumbria by this time had become the center of learning. The Venerable Bede was ensonced at Jarrow. Other scholars were making a name for Northumbria as well. So when a place was desired for an important religious conference, Whitby, in Northumbria, was chosen.
The Synod of Whitby marked a turning point in the teachings of Christianity in Britain. At this council, religious leaders decided to follow Roman, not Celtic, teachings. What really happened is that the practicers of Celtic Christianity suddenly had to stop doing things their own way and follow the lead of Rome. Absent Roman direction for a few hundred years in the period between the Roman withdrawal (in 410) and the Roman reappearance (in the form of Augustine in 596), the Celtic Christians had kept the teachings of Patrick and Columba and had adapted their religious practices to fit the audience.
But after Whitby, Christian leaders followed the Roman line. Celtic Christianity did not die, however; it just faded back into Ireland and Scotland, there to exist in isolation from the Saxon occupation of England.