2 In the first place, let men preparing for sacred orders be taught the best things, those for which there is the greatest need and which will enable them to teach Catholic truths and to defend them strenuously against all assaults. All too often daily experience makes it plain that where the ministers lack appropriate doctrinal knowledge, their people generally suffer from ignorance of the faith and religion. For it is from the mouth of the priest that the faithful ought to learn the law: He is the angel of the Lord. For this reason we read the proclamation: the lips of the priest shall keep knowledge.(1) The Apostle too mentions knowledge among other reasons for proving himself your servant in Jesus.(2) And where this knowledge is wanting, this evil also follows for the priests: they are condemned by the people, with God also exacting a penalty for their neglect of duty. Therefore have I also made you contemptible and base before all people.(3) - But the embellishment of learning and its defense never leads to their goal if they are separated from sanctity of life and morals. Indeed, knowledge without love does not build up but inflates.(4) This is the usual practice of man. Though Christ taught that learning is to be accepted from the sacred ministers without regard for any deeds of theirs which do not conform to the doctrine; nevertheless men are more influenced by what they see than by what they hear. For this reason we read the clear testimony concerning God the Saviour, who was not only the teacher of shepherds but also became their model, that he began to do and to teach. Hence the priest's actions must confirm the doctrine which he preaches and commends. Before all else he who is appointed to rule a parish must not be impatient with the labor. Called to the vineyard of the Lord, let him diligently cultivate it, aware that he must render a serious account sometime for the souls entrusted to him. Nor does he labor in vain if at all times and in all matters he holds fast to learning. We must indeed fight strenuously for Christ, but only at the will and authorization of those whom Christ has chosen as leaders.
Source: Litteras a Vobis (Vatican.va)