The big difference between the Roman Catholic and Celtic Christianity led to the Synod of Whitby in 664. The decisions that were taken, were detrimental to the Celts. The Irish monastic rules were replaced with the rules of the Benedictines and strict adherence to Catholic doctrine was enforced. The decree of Whitby had no immediate effect. Especially Devon, Cornwall and Scotland continued to protest against the new form of Christianity. This resulted in the presence of a Celtic monastery on Iona (Scotland) until the 13th century, which then was replaced by a Benedictine abbey. However, Christianity in Britain began to adapt increasingly to the Roman Catholicism. Despite this, Celtic Christianity was passed on orally and there have always remained elements of the former belief in British and Irish churches. After the Reformation of the 16th century the Celtic tradition got offered more and more resistance. Reading the prayers out loud was discouraged and even forbidden, because it was thought that this was a pagan and polytheistic origin. In Scotland a combination of religious persecution and the highland clearances led to the weakening of the Celtic culture. But even this did not lead to the end of the Celtic Christianity. In the early 20th century Celtic prayers were collected in Gaelic and written down due to a resurgence of interest in Celtic literature. Partly because of this there was a growing interest in Celtic Christianity. People protested less against the tradition of "pagan" elements and more people began to appreciate the religion.