Early Insular Christianity is a term used to cover Christianity in Great Britain

Great Britain

Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

 and Ireland

Ireland

Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...

 during the post-Roman period

Sub-Roman Britain

Sub-Roman Britain is a term derived from an archaeological label for the material culture of Britain in Late Antiquity: the term "Sub-Roman" was invented to describe the potsherds in sites of the 5th century and the 6th century, initially with an implication of decay of locally-made wares from a...

. It splits into two strands:

• Celtic Christianity
  Celtic Christianity
  Celtic Christianity or Insular Christianity refers broadly to certain features of Christianity that were common, or held to be common, across the Celtic-speaking world during the Early Middle Ages...
  
   (5th to 8th centuries), the Christianity surviving from the Roman period in the Celtic areas of Britain and Ireland (e.g. Ireland, Wales, Scotland) and spread to parts of Anglo-Saxon England by the Hiberno-Scottish mission
  Hiberno-Scottish mission
  The Hiberno-Scottish mission was a mission led by Irish and Scottish monks which spread Christianity and established monasteries in Great Britain and continental Europe during the Middle Ages...
  
  
• Anglo-Saxon Christianity (7th to 11th centuries), the Christianity imported from Rome by Augustine of Canterbury
  Augustine of Canterbury
  Augustine of Canterbury was a Benedictine monk who became the first Archbishop of Canterbury in the year 597...
  
   in 597

The Synod of Whitby

Synod of Whitby

The Synod of Whitby was a seventh century Northumbriansynod where King Oswiu of Northumbria ruled that his kingdom would calculate Easter and observe the monastic tonsure according to the customs of Rome, rather than the customs practised by Iona and its satellite institutions...

 of 664

664

Year 664 was a leap year starting on Monday of the Julian calendar. The denomination 664 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.- Asia :* Arab armies conquer Kabul.* Kuo Wu Tsung of Tang...

 forms a significant watershed in that King Oswiu of Northumbria

Oswiu of Northumbria

Oswiu , also known as Oswy or Oswig , was a King of Bernicia. His father, Æthelfrith of Bernicia, was killed in battle, fighting against Rædwald, King of the East Angles and Edwin of Deira at the River Idle in 616...

 decided to follow Roman rather than Celtic practices, but the two rites co-existed for another century, Wales observing the Celtic Easter date until 768

768

Year 768 was a leap year starting on Friday of the Julian calendar. The denomination 768 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.- Europe :* Pepin the Short , king of the Franks since...

. The Anglo-Saxon mission

Anglo-Saxon mission

Anglo-Saxon missionaries were instrumental in the spread of Christianity in the Frankish Empire during the 8th century, continuing the work of Hiberno-Scottish missionaries which had been spreading Celtic Christianity across the Frankish Empire as well as in Scotland and Anglo-Saxon England itself...

in the 7th to 8th centuries spread Christianity to the Continent, as had the earlier Celtic missions.