St. Maximus the Confessor (c. 650)
A celebrated theologian and a native of Constantinople
The extremities of the earth, and everyone in every part of it who purely and rightly confess the Lord, look directly towards the Most Holy Roman Church and her confession and faith, as to a sun of unfailing light awaiting from her the brilliant radiance of the sacred dogmas of our Fathers, according to that which the inspired and holy Councils have stainlessly and piously decreed. For, from the descent of the Incarnate Word amongst us, all the churches in every part of the world have held the greatest Church alone to be their base and foundation, seeing that, according to the promise of Christ Our Savior, the gates of hell will never prevail against her, that she has the keys of the orthodox confession and right faith in Him, that she opens the true and exclusive religion to such men as approach with piety, and she shuts up and locks every heretical mouth which speaks against the Most High. (Maximus, Opuscula theologica et polemica, Migne, Patr. Graec. vol. 90)
How much more in the case of the clergy and Church of the Romans, which from old until now presides over all the churches which are under the sun? Having surely received this canonically, as well as from councils and the apostles, as from the princes of the latter (Peter and Paul), and being numbered in their company, she is subject to no writings or issues in synodical documents, on account of the eminence of her pontificate .....even as in all these things all are equally subject to her (the Church of Rome) according to sacerodotal law. And so when, without fear, but with all holy and becoming confidence, those ministers (the popes) are of the truly firm and immovable rock, that is of the most great and Apostolic Church of Rome. (Maximus, in J.B. Mansi, ed. Amplissima Collectio Conciliorum, vol. 10)
John VI, Patriarch of Constantinople (715)
The Pope of Rome, the head of the Christian priesthood, whom in Peter, the Lord commanded to confirm his brethren. (John VI, Epist. ad Constantin. Pap. ad. Combefis, Auctuar. Bibl. P.P. Graec.tom. ii. p. 211, seq.)
St. Nicephorus, Patriarch of Constantinople (758-828)
Without whom (the Romans presiding in the seventh Council) a doctrine brought forward in the Church could not, even though confirmed by canonical decrees and by ecclesiastical usuage, ever obtain full approval or currency. For it is they (the Popes of Rome) who have had assigned to them the rule in sacred things, and who have received into their hands the dignity of headship among the Apostles. (Nicephorus, Niceph. Cpl. pro. s. imag. c 25 [Mai N. Bibl. pp. ii. 30]).
St. Theodore the Studite of Constantinople (759-826)
Writing to Pope Leo III:
Since to great Peter Christ our Lord gave the office of Chief Shepherd after entrusting him with the keys of the Kingdom of Heaven, to Peter or his successor must of necessity every novelty in the Catholic Church be referred. [Therefore], save us, oh most divine Head of Heads, Chief Shepherd of the Church of Heaven. (Theodore, Bk. I. Ep. 23)
Writing to Pope Paschal:
Hear, O Apostolic Head, divinely-appointed Shepherd of Christ's sheep, keybearer of the Kingdom of Heaven, Rock of the Faith upon whom the Catholic Church is built. For Peter art thou, who adornest and governest the Chair of Peter. Hither, then, from the West, imitator of Christ, arise and repel not for ever (Ps. xliii. 23). To thee spake Christ our Lord: 'And thou being one day converted, shalt strengthen thy brethren.' Behold the hour and the place. Help us, thou that art set by God for this. Stretch forth thy hand so far as thou canst. Thou hast strength with God, through being the first of all. (Letter of St. Theodore and four other Abbots to Pope Paschal, Bk. ii Ep. 12, Patr. Graec. 99, 1152-3)
Writing to Emperor Michael:
Order that the declaration from old Rome be received, as was the custom by Tradition of our Fathers from of old and from the beginning. For this, O Emperor, is the highests of the Churches of God, in which first Peter held the Chair, to whom the Lord said: Thou art Peter ...and the gates of hell shall not prevail against it. (Theodore, Bk. II. Ep. 86)
Sergius, Metropolitain of Cyprus (649)
Writing to Pope Theodore:
O Holy Head, Christ our God hath destined thy Apostolic See to be an immovable foundation and a pillar of the Faith. For thou art, as the Divine Word truly saith, Peter, and on thee as a foundation-stone have the pillars of the Church been fixed. (Sergius Ep. ad Theod. lecta in Sess. ii. Concil. Lat. anno 649)