24 Bishops, as successors of the apostles, receive from the Lord, to whom was given all power in heaven and on earth, the mission to teach all nations and to preach the Gospel to every creature, so that all men may attain to salvation by faith, baptism and the fulfilment of the commandments. To fulfill this mission, Christ the Lord promised the Holy Spirit to the Apostles, and on Pentecost day sent the Spirit from heaven, by whose power they would be witnesses to Him before the nations and peoples and kings even to the ends of the earth. And that duty, which the Lord committed to the shepherds of His people, is a true service, which in sacred literature is significantly called "diakonia" or ministry. The canonical mission of bishops can come about by legitimate customs that have not been revoked by the supreme and universal authority of the Church, or by laws made or recognized be that the authority, or directly through the successor of Peter himself; and if the latter refuses or denies apostolic communion, such bishops cannot assume any office.
Source: Lumen Gentium (Vatican.va)