2 Our Predecessor St. Celestine I hailed him as 'good defender of the Catholic faith,' as 'excellent priest,' as 'apostolic man.' The ecumenical Council of Chalcedon not only used his doctrine for the detecting and refuting of the latest errors, but went so far as to compare it with the learning of St. Leo the Great; and in fact the latter praised and commended the writings of this great Doctor because of their perfect agreement with the faith of the holy Fathers. The fifth ecumenical Council, held at Constantinople, treated St. Cyril's authority with similar reverence and many years later, during the controversy about the two wills in Christ, his teaching was rightly and triumphantly vindicated, both in the first Lateran Council and in the sixth ecumenical Council, against the false charge of being tainted with the error of Monothelitism. He was, as Our saintly Predecessor Agatho proclaimed, 'a defender of the truth' and 'a consistent teacher of the orthodox faith.'
Source: Orientalis Ecclesiae (Vatican.va)