11 A diocese is a portion of the people of God which is entrusted to a bishop to be shepherded by him with the cooperation of the presbytery. Thus by adhering to its pastor and gathered together by him through the Gospel and the Eucharist in the Holy Spirit, it constitutes a particular church in which the one, holy, catholic, and apostolic Church of Christ is truly present and operative. Individual bishops who have been entrusted with the care of a particular church-under the authority of the supreme pontiff-feed their sheep in the name of the Lord as their own, ordinary, and immediate pastors, performing for them the office of teaching, sanctifying, and governing. Nevertheless, they should recognize the rights which legitimately belong to patriarchs or other hierarchical authorities. Bishops should dedicate themselves to their apostolic office as witness of Christ before all men. They should not only look after those who already follow the Prince of Pastors but should also wholeheartedly devote themselves to those who have strayed in any way from the path of truth or are ignorant of the Gospel of Christ and His saving mercy until finally all men walk "in all goodness and justice and truth" (Eph. 5:9).
Source: Christus Dominus (Vatican.va)