During the early 400s, Rome had to withdraw its soldiers from the island of Britain to help defend itself against the invasions by various barbarian groups. This left the native Romano-British vulnerable to attacks from invaders from all around. In order to defend themselves, the Romano-British hired Saxon mercenaries to fight off the attacking forces. This plan backfired when the Saxons seized this opportunity to steal land from the Romano-British, forcing them into the north-western areas of Britain. After this, the native Britons were so angry with the Saxons that they would not try to convert them.
Pope Gregory the Great (r. 590-604) decided that he needed to take action in the conversion of the Anglo-Saxons, and sent Augustine of Canterbury to head a mission to England in 596. Upon their arrival in 597, the 40 missionaries went to the chief King, Æthelberht of Kent (r. 589-616) of to seek his approval. They received it, and had great success with the conversions, including Æthelberht himself around 597. After the death of Æthelberht, missionary work in England was thrown into confusion.
The Benedictine missionaries sent by Gregory the Great weren’t the only missionaries operating in England. The Irish, who had been single-handedly converted to Christianity by St. Patrick in the 5th century, had been sending missionaries as well. The Irish Church had different traditions than the Roman Church, like the dating of Easter, but more importantly the system of hierarchy among the clergy. Unlike in the Roman Church, authority came not from the Bishops, but from the Abbots of monasteries. Irish monks also practiced extreme asceticism, which was at odds with traditional Benedictine monasticism. Because of this, the Anglo-Saxons were being converted to two different types Catholicism.
Finally, things were resolved between the Irish and Benedictine traditions in 664 at the Synod Whitby. When King Oswiu of Northumbria (r. 642-670) heard at the Synod that the dating of Easter and other Benedictine traditions came from the Pope, he decreed that the traditions of the Roman church were the legitimate ones to be followed, and in so doing resolved the differences between the Roman and Irish traditions.