I have begun reading the British legal historian Theodore F.T. Plucknett’s massive work, A Concise History of the Common Law, a wonderful treatment of the subject written in the mid-twentieth century. Here’s something from early in the book:

While imperial Rome was slowly declining, Christianity was entering on a period of remarkable growth. At first it was hardly noticed among the numerous new cults which were fashionable importations from the Near East, some of which were extremely popular. After being ignored, it was later persecuted, then under the great Constantine it was at last tolerated (324). So far, the established “Hellenistic” religion had been considered as an official department, and its priests as civil servants. Attempts had been made to incorporate with it the religions of Isis, Mithras, Christ, and others, on a similar footing, combining all the known gods in one vast polytheism, whose cult was to be maintained and controlled by the State. It was soon evident, however, that Christianity would not accept this inferior position. Although some things were Caesar’s, others were God’s, and from this fundamental conflict arose the problem of Church and State, which has lasted from Constantine’s day to our own. The controversy took a variety of forms in the course of the succeeding sixteen centuries. Stated in its broadest and most general terms, it means that many earnest thinkers find it impossible to accept the State as the highest form of human society, and that they recognize some situations in which they would feel bound to obey some other duty than that imposed by the State. On the continent it lay at the root of the long conflict between the Empire and the papacy; in England it took such varied forms as the conflict with Thomas Becket, the discussion in Bracton as to the real position of the King (who is subject, he says, to God “and the law”), the Puritan revolution–and may even be traced in the American constitutions, for the modern attempts to curb the power of the State by means of constitutional limitations are the result of the same distrust of the State as was expressed in former days in the conflict between religion and the secular power.

It was also during the reign of Constantine that the great Council of Nicaea was held (325), attended by almost three hundred bishops from all parts of the world. Besides settling many fundamental matters of doctrine, this council gave an imposing demonstration of the world-wide organisation of the Church, and from this point onwards that organisation grew increasingly effective, and the Church became more and more a world power. As a result, the Empire had to admit the presence first of a potent ally, and soon of a vigorous rival.

The Nicene canons are the earliest code that can be called canon law of the whole Church, and at least in the West they enjoyed something like the same finality in the realm of discipline that the Nicene Creed enjoyed in the realm of doctrine. [citing C.H. Turner, Cambridge Mediaeval History]

Indeed, while the organization of the Empire was slowly breaking down, that of the Church was steadily growing, with the result that the Church soon offered a career comparable to, if not better than, that afforded by the State to men of ability who felt called to public life. Some specialised in the study of theology; others took up the work of creating the great body of canon law which for a long time was to perpetuate the old Roman ideal of universal law. With all this, the growth of the episcopate, and particularly of the papacy, was to give a new aspect to the ancient city of Rome, and slowly, but certainly, the Empire ruled from Rome was being replaced for many purposes by Christendom ruled by the papacy. [4-5]