Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Sixth Ecumenical Council.  The Third Council of Constantinople.: The Letter of Agatho and of the Roman Synod of 125 Bishops which was to Serve as an Instruction to the Legates Sent to Attend the Sixth Synod.

p. 340 The Letter of Agatho and of the Roman Synod of 125 Bishops which was to Serve as an Instruction to the Legates Sent to Attend the Sixth Synod.

(Found in Labbe and Cossart, Concilia, Tom. VI., col. 677 et seqq., and in Migne, Pat. Lat. Tom. LXXXVII., col. 1215 et seqq.  [This last text, which is Mansi’s, I have followed].)

To the most pious Lords and most serene victors and conquerors, our own sons beloved of God and of our Lord Jesus Christ, Constantine, the great Emperor, and Heraclius and Tiberius, Augustuses, Agatho, the bishop and servant of the servants of God, together with all the synods subject to the council of the Apostolic See.

[The Letter opens with a number of compliments to the Emperor, much in style and matter like the introduction of the preceding letter.  I have not thought it worth while to translate this, but have begun at the doctrinal part, which is given to the reader in full.  (Labbe and Cossart, col. 682.)]

We believe in God the Father Almighty, maker of heaven and earth, and of all things visible and invisible; and in his only-begotten Son, who was begotten of him before all worlds; very God of Very God, Light of Light, begotten not made, being of one substance with the Father, that is of the same substance as the Father; by him were all things made which are in heaven and which are in earth; and in the Holy Ghost, the Lord and giver of life, who proceedeth from the Father, and with the Father and the Son together is worshipped and glorified; the Trinity in unity and Unity in trinity; a unity so far as essence is concerned, but a trinity of persons or subsistences; and so we confess God the Father, God the Son, and God the Holy Ghost; not three gods, but one God, the Father, the Son, and the Holy Ghost:  not a subsistency of three names, but one substance of three subsistences; and of these persons one is the essence, or substance or nature, that is to say one is the godhead, one the eternity, one the power, one the kingdom, one the glory, one the adoration, one the essential will and operation of the same Holy and inseparable Trinity, which hath created all things, hath made disposition of them, and still contains them.

Moreover we confess that one of the same holy consubstantial Trinity, God the Word, who was begotten of the Father before the worlds, in the last days of the world for us and for our salvation came down from heaven, and was incarnate of the Holy Ghost, and of our Lady, the holy, immaculate, ever-virgin and glorious Mary, truly and properly the Mother of God, that is to say according to the flesh which was born of her; and was truly made man, the same being very God and very man.  God of God his Father, but man of his Virgin Mother, incarnate of her flesh with a reasonable and intelligent soul:  of one substance with God the Father, as touching his godhead, and consubstantial with us as touching his manhood, and in all points like unto us, but without sin.  He was crucified for us under Pontius Pilate, he suffered, was buried and rose again; ascended into heaven, and sitteth at the right hand of the Father, and he shall come again to judge both the quick and the dead, and of his kingdom there shall be no end.

And this same one Lord of ours, Jesus Christ, the only-begotten Son of God, we acknowledge to subsist of and in two substances unconfusedly, unchangeably, indivisibly, inseparably, the difference of the natures being by no means taken away by the union, but rather the proprieties of each nature being preserved and concurring in one Person and one Subsistence, not scattered or divided into two Persons, nor confused into one composite nature; but we confess one and the same only-begotten Son, God the Word, our Lord Jesus Christ, not one in another, nor one added to another, but himself the same in two natures—that is to say in the Godhead and in the manp. 341 hood even after the hypostatic union:  for neither was the Word changed into the nature of flesh, nor was the flesh transformed into the nature of the Word, for each remained what it was by nature.  We discern by contemplation alone the distinction between the natures united in him of which inconfusedly, inseparably and unchangeably he is composed; for one is of both, and through one both, because there are together both the height of the deity and the humility of the flesh, each nature preserving after the union its own proper character without any defect; and each form acting in communion with the other what is proper to itself.  The Word working what is proper to the Word, and the flesh what is proper to the flesh; of which the one shines with miracles, the other bows down beneath injuries.  Wherefore, as we confess that he truly has two natures or substances, viz.:  the Godhead and the manhood, inconfusedly, indivisibly and unchangeably [united], so also the rule of piety instructs us that he has two natural wills and two natural operations, as perfect God and perfect man, one and the same our Lord Jesus Christ.  And this the apostolic and evangelical tradition and the authority of the Holy Fathers (whom the Holy Apostolic and Catholic Church and the venerable Synods receive), has plainly taught us.

[The letter goes on to say that this is the traditional faith, and is that which was set forth in a council over which Pope Martin presided, and that those opposed to this faith have erred from the truth, some in one way, and some in another.  It next apologizes for the delay in sending the persons ordered by the imperial Sacra, and proceeds thus:  (Labbe and Cossart, col. 686; Migne, col. 1224).]

In the first place, a great number of us are spread over a vast extent of country even to the sea coast, and the length of their journey necessarily took much time.  Moreover we were in hopes of being able to join to our humility our fellow-servant and brother bishop, Theodore, the archbishop and philosopher of the island of Great Britain, with others who have been kept there even till to-day; and to add to these divers bishops of this council who have their sees in different parts, that our humble suggestion [i.e., the doctrinal definition contained in the letters] might proceed from a council of wide-spread influence, lest if only a part were cognizant of what was being done, it might escape the notice of a part; and especially because among the Gentiles, as the Longobards, and the Sclavi, as also the Franks, the French, the Goths, and the Britains, there are known to be very many of our fellow-servants who do not cease curiously to enquire on the subject, that they may know what is being done in the cause of the Apostolic faith:  who as they can be of advantage so long as they hold the true faith with us, and think in unison with us, so are they found troublesome and contrary, if (which may God forbid!) they stumble at any article of the faith.  But we, although most humble, yet strive with all our might that the commonwealth of your Christian empire may be shown to be more sublime than all the nations, for in it has been founded the See of Blessed Peter, the prince of the Apostles, by the authority of which, all Christian nations venerate and worship with us, through the reverence of the blessed Apostle Peter himself.  (This is the Latin, which appears to me to be corrupt, the Greek reads as follows:  “The authority of which for the truth, all the Christian nations together with us worship and revere, according to the honour of the blessed Peter the Apostle himself.”)

[The letter ends with prayers for constancy, and blessings on the State and Emperor, and hopes for the universal diffusion and acceptance of the truth.]