The following is John Collorafi's translation of Agatho's letter which was read aloud during the fourth session of the Third Council of Constantinople (680/681), translated from volume 11 of Mansi, Sacrorum Conciliorum nova et amplissima Collectio:
This is the apostolic and evangelical tradition
which your most felicitous empire’s spiritual mother, the apostolic church of
Christ holds. This is the true and immaculate profession of the Christian
religion, which human ingenuity did not invent, but which the princes of the
apostles taught . . . which Blessed Peter the apostle . . . taught, not that it
might be hidden under a bushel, but that it be preached more loudly than a
trumpet throughout the world: because his true confession was revealed by the
Heavenly Father, for which Peter was pronounced blessed by the Lord of all.
With a threefold recommendation, [Peter] also received from the very Redeemer
of all, the Church’s spiritual sheep, to feed them: by those protection, this
apostolic Church of his has never turned aside to any part of error, whose
authority, as that of the prince of all the apostles, the entire Catholic
Church of Christ, and the ecumenical councils, ever faithfully embracing,
followed in all things . . . this is the rule of the truth faith, which both in
prosperity and adversity your most serene empire’s spiritual mother, the apostolic
church of Christ, has keenly held and defended: which, by the grace of Almighty
God, shall be shown never to have erred from the path of apostolic tradition,
nor has she succumbed to being corrupted by heretical innovations, but as he
has received from the beginning of the Christian faith from her authors, the
princes of Christ’s apostles, she remains unblemished to the very end, in
accordance with the divine promise of the very Lord (and) Savior, which in the
sacred Gospels he uttered to the prince of his disciples, saying: “Peter,
Peter, behold Satan has sought to sift you, as one who sifts wheat: but I have
prayed for thee, that thy faith may not fail, and do you, when you have been
converted, strengthen your brethren.”
Let your Serene Clemency consider, therefore, that
the Lord and Savior of all, from whom the faith comes, who promised that the
faith of Peter would not fail, admonished him to strengthen his brethren:
which, as everybody knows, the apostolic pontiffs, predecessors of my
lowliness, ever did with confidence . . . For woe to me, if I neglect to preach
what they preached with sincerity. Woe to me, if I cover over in silence what I
have been commanded to instill into and to teach the Christian people. (Mansi
11:239-42) (Scott Butler and John Collorafi, Keys Over the Christian World:
The Evidence for Papal Authority (33 AD – 800 AD) from Ancient Latin, Greek,
Chaldean, Syriac, Armenian, Coptic and Ethiopian Documents [State Line,
Pa.: Catholic Apologetics International Publishing, Inc., 2021], 364-66)