9 Here then it is well to emphasize and insist that for a priest there is no duty more grave or obligation more binding than this. Who, indeed, will deny that knowledge should be joined to holiness of life in the priest? "For the lips of the priest shall keep knowledge." The Church demands this knowledge of those who are to be ordained to the priesthood. Why? Because the Christian people expect from them knowledge of the divine law, and it was for that end that they were sent by God. "And they shall seek the law at his mouth; because he is the angel of the Lord of hosts." Thus the bishop speaking to the candidates for the priesthood in the ordination ceremony says: "Let your teaching be a spiritual remedy for God's people; may they be worthy fellow-workers of our order; and thus meditating day and night on His law, they may believe what they read, and teach what they shall believe."
Source: Acerbo Nimis (Vatican.va)