94 This service of unity, rooted in the action of divine mercy, is entrusted within the College of Bishops to one among those who have received from the Spirit the task, not of exercising power over the people—as the rulers of the Gentiles and their great men do (cf. Mt 20:25; Mk 10:42)—but of leading them towards peaceful pastures. This task can require the offering of one's own life (cf. Jn 10:11-18). Saint Augustine, after showing that Christ is "the one Shepherd, in whose unity all are one", goes on to exhort: "May all shepherds thus be one in the one Shepherd; may they let the one voice of the Shepherd be heard; may the sheep hear this voice and follow their Shepherd, not this shepherd or that, but the only one; in him may they all let one voice be heard and not a babble of voices ... the voice free of all division, purified of all heresy, that the sheep hear". The mission of the Bishop of Rome within the College of all the Pastors consists precisely in "keeping watch" ( episkopein ), like a sentinel, so that, through the efforts of the Pastors, the true voice of Christ the Shepherd may be heard in all the particular Churches. In this way, in each of the particular Churches entrusted to those Pastors, the una, sancta, catholica et apostolica Ecclesia is made present. All the Churches are in full and visible communion, because all the Pastors are in communion with Peter and therefore united in Christ. With the power and the authority without which such an office would be illusory, the Bishop of Rome must ensure the communion of all the Churches. For this reason, he is the first servant of unity. This primacy is exercised on various levels, including vigilance over the handing down of the Word, the celebration of the Liturgy and the Sacraments, the Church's mission, discipline and the Christian life. It is the responsibility of the Successor of Peter to recall the requirements of the common good of the Church, should anyone be tempted to overlook it in the pursuit of personal interests. He has the duty to admonish, to caution and to declare at times that this or that opinion being circulated is irreconcilable with the unity of faith. When circumstances require it, he speaks in the name of all the Pastors in communion with him. He can also—under very specific conditions clearly laid down by the First Vatican Council— declare ex cathedra that a certain doctrine belongs to the deposit of faith. By thus bearing witness to the truth, he serves unity.
Source: Ut Unum Sint (Vatican.va)