The monastery itself became the center of Celtic Christianity. Iona was a major supporter of the "Celtic" system for calculating the date of Easter. The missionaries and bishops who came to Britain from Rome used a Roman calculation for the date of Easter. This issue was settled when they adopted the Roman system at the Synod of Whitby in 664. Pictland adopted the Roman system in the early 8th century. Iona changed systems in 716, according to the Anglo-Saxon historian Bede. Iona's prominence was reduced over the next centuries as a result of Viking raids and the rise of other powerful monasteries in the system, such as the Abbey of Kells. The Book of Kells was created on Iona about the year 800. At about the same time the island's high crosses were erected. They had a ring around the top part that became the symbol called a "Celtic cross". The series of Viking raids on Iona began in 794 and, after its treasures had been plundered many times, Columba's relics were removed and divided two ways between Scotland and Ireland in 849 as the monastery was abandoned.