In Letter 162, Pope St. Leo wrote the following, addressing Leo Augustus (the then-Roman Emperor):
Although,
therefore, I am very confident of the piety of your heart in all things, and
perceive that through the Spirit of God dwelling in you, you are sufficiently
instructed, nor can any error delude your faith, yet I will endeavour to follow
your bidding so far as to send certain of my brothers to represent my person
before you, and to set forth what the Apostolic rule of Faith is, although, as
I have said, it is well known to you, in all things making it clear and certain
that they are not in any way to be reckoned among catholics, who do not accept
the definitions of the venerable Synod of Nicaea or the ordinances of the holy
Council of Chalcedon, inasmuch as it is evident the holy decrees of both
proceed from the Evangelical and Apostolical source, and whatever is not of
Christ's watering is like a snake-poisoned draught. Your Majesty should
understand beforehand, most venerable Emperor, that those whom I undertake to
send will come from the Apostolic See, not to fight with the enemies of the
Faith nor to strive against any, because of matters already settled as it has
pleased God both at Nicaea and at Chalcedon we dare not enter upon any
discussion, as if what so great an authority has fixed by the Holy Spirit were
doubtful or weak. (Letter 162.3, NPNF2 12:105)
If this was said by Augustus to Leo, it would be a favourite proof-text for papal infallibility in the first millennium; however, the Pontiff said this about the Emperor! It shows one must be cautious against proof-texting patristics when it comes to the Roman Pontiff (and yes, other issues, too, including LDS approaches to patristics and theosis).