1 Christ, the Eternal King, before he promised the headship of the Church to Peter, the son of John, called together his disciples, and asking them what they and other men believed about himself, praised the faith which would conquer all the storms and attacks of the evil powers, and which Peter, enlightened by the Eternal Father, had declared in these words: 'Thou art Christ, the Son of the Living God' ( Matt . XVI, 16). It is this faith which is 'the strength of God for the salvation of every believer' ( Rom . I, 16), and which brings forth the apostle's crown, the martyr's palm and the virgin's lily. This faith has been defended and lucidly clarified especially by three oecumenical councils, those of Nicea, Ephesus and Chalcedon. It is now fifteen hundred years since the last of these was concluded. It is fitting, therefore, that both at Rome and in the whole Catholic world, this most happy event should be celebrated with due solemnity; and so giving thanks to God the inspirer of all holy counsels, with deeply moved hearts, we institute those solemnities.
Source: Sempiternus Rex Christus (Vatican.va)