(during the fourth century) a branch of the Catholic Church of the West; and during the whole of that century, from the Council of Arles (316) onward, took part in all proceedings concerning the Church." After Roman troops withdrew from Britain, the "absence of Roman military and governmental influence and overall decline of Roman imperial political power enabled Britain and the surrounding isles to develop distinctively from the rest of the West. A new culture emerged around the Irish Sea among the Celtic peoples with Celtic Christianity at its core. What resulted was a form of Christianity distinct from Rome in many traditions and practices." The historian Charles Thomas, in addition to the Celticist Heinrich Zimmer, writes that the distinction between sub-Roman and post-Roman Insular Christianity, also known as Celtic Christianity, began to become apparent around AD 475, with the Celtic churches allowing married clergy, observing Lent and Easter according to their own calendar, and having a different tonsure; moreover, like the Eastern Orthodox and the Oriental Orthodox Churches, the Celtic churches operated independently of the Pope's authority, as a result of their isolated development in the British Isles. In what is known as the Gregorian mission, Pope Gregory I sent Augustine of Canterbury to the British Isles in AD 596, with the purpose of evangelising the pagans there (who were largely Anglo-Saxons), as well as to reconcile the Celtic churches in the British Isles to the See of Rome. In Kent, Augustine persuaded the Anglo-Saxon king "Æthelberht and his people to accept Christianity". Augustine, on two occasions, "met in conference with members of the Celtic episcopacy, but no understanding was reached between them." Eventually, the "Christian Church of the Anglo-Saxon kingdom of Northumbria convened the Synod of Whitby in 663/664 to decide whether to follow Celtic or Roman usages." This meeting, with King Oswiu as the final decision maker, "led to the acceptance of Roman usage elsewhere in England and brought the English Church into close contact with the Continent". As a result of assuming Roman usages, the Celtic Church surrendered its independence, and, from this point on, the Church in England "was no longer purely Celtic, but became Anglo-Roman-Celtic". The theologian Christopher L. Webber writes that, although "the Roman form of Christianity became the dominant influence in Britain as in all of western Europe, Anglican Christianity has continued to have a distinctive quality because of its Celtic heritage."