West: Bishop Hosius of Córdoba says in 353 [Letter to Emperor Constantius II in St. Athanasius the Great's History of the Arians 6:44],
Cease these proceedings, I beseech you, and remember that you are a mortal man. Be afraid of the day of judgment, and keep yourself pure thereunto. Intrude not yourself into Ecclesiastical matters, neither give commands unto us concerning them; but learn them from us. God has put into your hands the kingdom; to us He has entrusted the affairs of His Church; and as he who would steal the empire from you would resist the ordinance of God, so likewise fear on your part lest by taking upon yourself the government of the Church, you become guilty of a great offense. It is written, "Render unto Caesar the things that are Caesar's, and unto God the things that are God's" [Mt 22:21]. Neither therefore is it permitted unto us to exercise an earthly rule, nor have you, Sire, any authority to burn incense . These things I write unto you out of a concern for your salvation. With regard to the subject of your letters, this is my determination; I will not unite myself to the Arians; I anathematize their heresy. Neither will I subscribe against Athanasius, whom both we and the Church of the Romans and the whole Council pronounced to be guiltless.West: Bishop St. Lucifer of Cagliari says in 354 [Apology for St. Athanasius to Emperor Constantius II 1 in PL 13:826BC],
Prove to me that you have been appointed judge over us. Prove that you [Emperor Constantius II] have been appointed emperor in order that you may compel us, by your arms, to fulfill the will of your friend, the Devil. As you cannot prove this, because you are commanded, not only not to tyrannize over the bishops, but so to obey their commands, that if you should endeavor to subvert their decrees, if seized in your pride, you should be called to die, how could you say, that you can judge bishops, whom unless you obey, you shall be punished by God in a heavy pain of death.West: Bishop St. Ambrose the Great of Milan (Doctor) says [Epistle 21:4 to Emperor Valentinian I in PL 16:1004A], "But undoubtedly, whether we go through the series of the holy Scriptures, or the times of old, who is there who can deny that, in a matter of faith--in a matter I say of faith--bishops are wont to judge of Christian emperors, not emperors of bishops?"
The same courageous confessor says [Sermon Against Auxentius on the Giving Up of the Basilicas 36 in PL ], "For the Emperor is within the Church, not above it. For a good emperor seeks the aid of the Church and does not refuse it. As I say this with all humility, so also I state it with firmness. Some threaten us with fire, sword, exile; we have learned as servants of Christ not to fear."
West: Pope St. Innocent I of Rome says [Epistle 8:1 to Arcadius in PL 20:508B], "If, in the cause of religion, there should be any difference amongst the bishops, the decision should come from the bishops. For to them belongs the interpretation of Divine things."
East: St. Maximus the Confessor of Constantinople says [The Life of Our Holy Monastic Father Maximus the Confessor and Martyr], responding to Bishop Theodosius's statement that "It is the Emperor's summons that gives authority to a council":
If that were so, the Orthodox faith would have long since come to an end. Recall the councils summoned by imperial decree to proclaim that the Son of God is not of the same essence as God the Father. The first was held in Tyre, the second in Antioch, the third in Seleucia, the fourth in Constantinople under Eudoxius the Arian, the fifth in Nicaea, and the sixth in Sirmium. Considerably later, a seventh false council took place in Ephesus, at which Dioscorus presided. All these synods were convened by imperial decree, but were rejected and anathematized, since they endorsed godless doctrines. On what grounds, I would like to know, do you accept the council which condemned and anathematized Paul of Samosata? Gregory the Wonderworker presided over that council, and its resolutions were confirmed by Dionysius, Pope of Rome, and Dionysius of Alexandria. No Emperor convoked it, but it is unassailable and irrefutable. The Orthodox Church recognizes as true and holy precisely those synods that proclaimed true dogmas. Your holiness knows that the canons require that local councils be held twice yearly in every Christian land for the defense of our saving faith and for administrative purposes; however, they say nothing about imperial decrees.8th Century
West: Pope St. Gregory II of Rome says [Epistle 13 On Sacred Images to Emperor Leo III the Isaurian in PL 89:522BC],
For as the priest has not power to enter the palace, and to distribute royal honors, so the prince has not power to enter the Church to make decrees amongst the clergy, to offer sacrifice, or to touch the symbols of the sacred mysteries, nor to participate thereof, without the priest. Let each of us remain in the calling in which we have been called by God [1 Cor 7:20].Ecumenical Councils
Ecumenical Council of Nicaea II says in 787 [Canon 3],
Let every election of a bishop, presbyter, or deacon, made by princes stand null, according to the canon which says: If any bishop making use of the secular powers shall by their means obtain jurisdiction over any church, he shall be deposed, and also excommunicated, together with all who remain in communion with him. For he who is raised to the episcopate must be chosen by bishops, as was decreed by the holy fathers of Nice in the canon which says: It is most fitting that a bishop be ordained by all the bishops in the province; but if this is difficult to arrange, either on account of urgent necessity, or because of the length of the journey, three bishops at least having met together and given their votes, those also who are absent having signified their assent by letters, the ordination shall take place. The confirmation of what is thus done, shall in each province be given by the metropolitan thereof.