49 As a service to the unity of faith and its integral transmission, the Lord gave his Church the gift of apostolic succession. Through this means, the continuity of the Church’s memory is ensured and certain access can be had to the wellspring from which faith flows. The assurance of continuity with the origins is thus given by living persons, in a way consonant with the living faith which the Church is called to transmit. She depends on the fidelity of witnesses chosen by the Lord for this task. For this reason, the magisterium always speaks in obedience to the prior word on which faith is based; it is reliable because of its trust in the word which it hears, preserves and expounds. In Saint Paul’s farewell discourse to the elders of Ephesus at Miletus, which Saint Luke recounts for us in the Acts of the Apostles, he testifies that he had carried out the task which the Lord had entrusted to him of "declaring the whole counsel of God" ( Acts 20:27). Thanks to the Church’s magisterium, this counsel can come to us in its integrity, and with it the joy of being able to follow it fully.
Source: Lumen Fidei (Vatican.va)