5 For the duty of preaching, as the Council of Trent teaches, "is the paramount duty of Bishops ." [Sess., xxiv, De. Ref., c.iv] And the Apostles, whose successors the bishops are, looked upon it as something peculiarly theirs. St. Paul writes: "For Christ sent us not to baptize, but to preach the Gospel. [I Cor . i:17] And the other Apostles were of the opinion that: "It is not reason that we should leave the Word of God and serve tables." [ Acts vi:2] But although preaching is properly the duty of Bishops, nevertheless, since it is impossible that they should always or everywhere be able to discharge it in person, distracted as they are by the many cares which they meet in the government of their churches, they must of necessity comply with this obligation through others. Wherefore it cannot be doubted 154 that all those who in addition to the Bishops are thus engaged, are employed in the performance of an episcopal duty. Let this then be the first law laid down: that no one on his own responsibility undertake the office of preaching. In order to fulfill that duty everyone must have a lawful mission, and that mission can be conferred by the Bishop alone. "How shall they preach, unless they be sent?" [ Rom . x:15] Now the Apostles were sent and sent by Him who is the supreme pastor and Bishop of our souls [I Pet . ii:25]; so too, were the seventy-two first disciples; nay, St. Paul himself, although constituted by Christ a vessel of election to carry His name, before Gentiles and kings, [ Acts ix:15] entered upon his apostolate only after the elders in obedience to the command of the Holy Ghost, "Separate me Saul for the work" [ Acts xiii:2], had imposed hands upon him and sent him forth. The same practice was constantly followed in the early days of the Church. For all without exception, both those who distinguished themselves in the priestly order like Origen, and those later on were raised to the dignity of the episcopate, like Cyril of Jerusalem, John Chrysostom, Augustine and the other more ancient Doctors of the Church, undertook the office of preaching with the sanction and authority of their Bishops.
Source: Humani Generis Redemptionem (Vatican.va)