Christianity had been the official imperial religion of the Roman Empire, and the first churches were built in England in the second half of the fourth century, overseen by a hierarchy of bishops and priests. Many existing pagan shrines were converted to Christian use and few pagan sites still operated by the fifth century. The collapse of the Roman system in the late fifth century, however, brought about the end of formal Christian religion in the east of England, and the new Germanic immigrants arrived with their own polytheistic gods, including Woden, Thunor and Tiw, still reflected in various English place names. Despite the resurgence of paganism in England, Christian communities still survived in more western areas such as Gloucestershire and Somerset.