Became the predominate religion in Europe, it linked all classes of people to a common belief system, which answered to the hierarchy of Rome known as the Pope (It was the be all and end all in Europe)
Adding to Johns answer. The Roman Catholic Church in many ways was a continuation of the Roman Empire. The Churches functions, regulations, hierarchy, etc. are almost identical to the former Roman systems of governance. So in effect, the Roman Empire never fell, and Caesar now goes by the title Pope. Two of the primary roles that the RCC played is 1) Acted as an interstate authority which superceded local rule. 2) Acted as a unifying force that instilled a sense of "nationalism" among all Catholic nations. This is how the Crusades were possible, and also allowed a man from Norway to have a sense of kinship with a man from Spain for example.