This is the end of Graeco-Roman heathenism, with its wisdom, and beauty. It fell a victim to a slow but steady process of incurable consumption. Its downfall is a sublime tragedy which, with all our abhorrence of idolatry, we cannot witness without a certain sadness. At the first appearance of Christianity it comprised all the wisdom, literature, art, and political power of the civilized world, and led all into the field against the weaponless religion of the crucified Nazarene. After a conflict of four or five centuries it lay prostrate in the dust without hope of resurrection.

Schaff, P., & Schaff, D. S. (1910). History of the Christian church (Vol. 3, p. 69). New York: Charles Scribner’s Sons.

Constantine, the first Christian Caesar, the founder of Constantinople and the Byzantine empire, and one of the most gifted, energetic, and successful of the Roman emperors, was the first representative of the imposing idea of a Christian theocracy, or of that system of policy which assumes all subjects to be Christians, connects civil and religious rights, and regards church and state as the two arms of one and the same divine government on earth. This idea was more fully developed by his successors, it animated the whole middle age, and is yet working under various forms in these latest times; though it has never been fully realized, whether in the Byzantine, the German, or the Russian empire, the Roman church-state, the Calvinistic republic of Geneva, or the early Puritanic colonies of New England. At the same time, however, Constantine stands also as the type of an undiscriminating and harmful conjunction of Christianity with politics, of the holy symbol of peace with the horrors of war, of the spiritual interests of the kingdom of heaven with the earthly interests of the state.

Schaff, P., & Schaff, D. S. (1910). History of the Christian church (Vol. 3, p. 12). New York: Charles Scribner’s Sons.

And this, in point of fact, took place first under Constantine, and developed under his successors, particularly under Justinian, into the system of the Byzantine imperial papacy, or of the supremacy of the state over the church.
Constantine once said to the bishops at a banquet, that he also, as a Christian emperor, was a divinely appointed bishop, a bishop over the external affairs of the church, while the internal affairs belonged to the bishops proper. In this pregnant word he expressed the new posture of the civil sovereign toward the church in a characteristic though indefinite and equivocal way. He made there a distinction between two divinely authorized episcopates; one secular or imperial, corresponding with the old office of Pontifex Maximus, and extending over the whole Roman empire, therefore ecumenical or universal; the other spiritual or sacerdotal, divided among the different diocesan bishops, and appearing properly in its unity and totality only in a general council.

Schaff, P., & Schaff, D. S. (1910). History of the Christian church (Vol. 3, pp. 133–134). New York: Charles Scribner’s Sons.

In general, from this time forth the prevailing view was, that God has divided all power between the priesthood and the kingdom (sacerdotium et imperium), giving internal or spiritual affairs, especially doctrine and worship, to the former, and external or temporal affairs, such as government and discipline, to the latter. But internal and external here vitally interpenetrate and depend on each other, as soul and body, and frequent reciprocal encroachments and collisions are inevitable upon state-church ground. This becomes manifest in the period before us in many ways, especially in the East, where the Byzantine despotism had freer play, than in the distant West.

The emperors after Constantine (as the popes after them) summoned the general councils, bore the necessary expenses, presided in the councils through commissions, gave to the decisions in doctrine and discipline the force of law for the whole Roman empire, and maintained them by their authority. The emperors nominated or confirmed the most influential metropolitans and patriarchs. They took part in all theological disputes, and thereby inflamed the passion of parties. They protected orthodoxy and punished heresy with the arm of power.

Schaff, P., & Schaff, D. S. (1910). History of the Christian church (Vol. 3, pp. 134–135). New York: Charles Scribner’s Sons.

After the defeat of the Arians by the second œcumenical Council, Theodosius the Great enforced uniformity of belief by legal penalties in fifteen edicts between 381 and 394. Honorius (408), Arcadius, the younger Theodosius, and Justinian (529) followed in the same path. By these imperial enactments heretics, i.e. open dissenters from the imperial state-religion, were deprived of all public offices, of the right of public worship, of receiving or bequeathing properly, of making binding contracts; they were subjected to fines, banishment, corporeal punishment, and even death. See the Theos. Code, Book XVI. tit. V. De Haereticis. The first sentence of death by the sword for heresy was executed on Priscillian and six of his followers who held Manichæan opinions (385). The better feeling of Ambrose of Milan and Martin of Tours protested against this act, but in vain. Even the great and good St. Augustin, although he had himself been a heretic for nine years, defended the principle of religious persecution, on a false exegesis of Cogite eos intrare, Luke 14:23 (Ep. 93 ad Vinc.; Ep. 185 ad Bonif., Retract. II. 5.). Had he foreseen the crusade against the Albigenses and the horrors of the Spanish Inquisition, he would have retracted his dangerous opinion. A theocratic or Erastian state-church theory—whether Greek Catholic or Roman Catholic or Protestant—makes all offences against the church offences against the state, and requires their punishment with more or less severity according to the prevailing degree of zeal for orthodoxy and hatred of heresy

Schaff, P., & Schaff, D. S. (1910). History of the Christian church (Vol. 2, pp. 515–516). New York: Charles Scribner’s Sons.