Together with those sacred writings of the all-wise Cyril, I likewise accept as being sacred and of equal honor, and the mother of the same Orthodoxy, also the God-given and divinely inspired letter of the great and illustrious Leo of godly mind, of the most holy Church of the Romans, or rather the luminary of all under the sun, which he wrote, clearly moved by the divine Spirit, to Flavian, the famous leader of the Queen of Cities, against the perverse Eutyches and Nestorius, hateful to God and deranged. Indeed I call and define this [letter] as ‘the pillar of orthodoxy’, following those holy Fathers who well defined it this way, as thoroughly teaching us every right belief, while destroying every heretical wrong belief, and driving it out of the halls of holy catholic church, guarded by God. With this divinely conceived epistle, and writing I also attach myself to all his letters and teachings as if they issued from the mouth of the chief Peter, and I kiss and cleave to them and embrace them with all my soul.
As I have said previously, I accept these five sacred and divine Councils of the blessed Fathers and all the writings of the all-wise Cyril, and especially those composed against the madness of Nestorius, and the epistle of the eastern leaders which was written to the most godly Cyril himself and which he attested as Orthodox. And [I accept] what Leo, the most holy shepherd of the most holy church of the Romans, wrote, and especially what he composed against the abomination of Eutyches and Nestorius. I recognize the latter as the definitions of Peter, the former those of Mark. (Synodical Letter 2.5.5, Sophronius of Jerusalem and Seventh-Century Heresy pp. 131-135)
Today we know the ‘Pillars of Orthodoxy’ to be the Saints that resisted western heresies (i.e. Papal Supremacy, Filioque, Barlaamism, Uniatism, etc.) however, in the past, the Holy Fathers of the Eastern Churches understood the ‘Pillar of Orthodoxy’ to be the Tome of St. Leo written against eastern christological heresies (Monophysitism and Nestorianism).