I was watching a documentary on the History Channel recently and they mentioned that when the Roman Empire converted to Christianity, they chose Catholicism because of its similarities to the hierarchy of the Roman government of the time. I had never heard that before and am wondering if there is any documented evidence of this being the main reason in their decision.
In my catechetical training, and in a biography of Constantine, I learned that the conversion of the Roman Empire was "top down". Constantine converted to Christianity, made Christians equal to those of practitioners of other religions, and lower class Romans found it expedient to adopt the religion of the emperor. The development of the hierarchy of Catholic Church resulted in similarities to the Roman political structures because the Catholic Church deliberately copied those structures because they worked. Romans at first did not convert but later decided to adopt Catholicism as their religion.
Constantine was looking for a model of unity which, because of its internal discipline, the Church appeared to provide. Paganism was disunited, with too many cults, and was far too tolerant to be a model for an autocratic empire. The alternative to Catholic-Orthodox Christianity was Gnostic Christianity, which made a virtue of disunity, refusing even to appoint bishops. Michael Grant says in The Emperor Constantine, page 161:
Apart from his deep emotional involvement, the main reason Constantine favoured Christianity was that he believed that it would encourage unity in the empire. Unfortunately the Christian faith failed to achieve that aim, due to the dissensions between one Christian group and another.
The Church hierarchy was not entirely similar to that of the Roman Empire as, for example, the Empire had a powerful leader, whereas the Church was led by an assemblage of bishops who had not yet really defined mechanisms for resolving differences among themselves -- deficiencies that Constantine attempted to resolve by calling the Council of Nicaea. Constantine had inherited the pagan title of Pontifex Maximus and now saw himself in this role as providing overall leadership for the Christian Church.
Constantine had discovered that in spite of the appearance of internal discipline, the Church was not united, with just the most recent controversies being Arianism and Donatism, but under his leadership the Church did become relatively unified. It was then able to help Constantine and his successors achieve their aim of managing the unwieldly empire, at least for the remainder of the fourth century.