17 Let us come now to the central point of the whole question, i.e. to the solemn definition of the Catholic faith, by which the pernicious error of Eutyches was rejected and condemned. In the fourth session of the sacred synod the representatives of the emperor asked that a new formula of the faith should be composed. But the papal legate, Paschasinus, expressed the feeling of all when he replied that it was not necessary; the ground, he said was sufficiently covered by the creeds already in use, and the canonical documents approved by the Church; among these the letter of St. Leo to Flavin was the most important. 'Thirdly (i.e., after the creeds of Nicea and Constantinople and their explanations by St. Cyril at the council of Ephesus) the writings composed by the holy and apostolic Leo, pope of the universal Church, against the heresies of Nestorius and Eutyches, have already shown what the true faith is. This holy synod likewise holds and follows this same faith' (Mansi, vii, 10 [Act. IV]).
Source: Sempiternus Rex Christus (Vatican.va)