57 But the portrait of the Catholic priest which we intend to exhibit to the world would be unfinished were We to omit another most important feature,--learning. This the Church requires of him; for the Catholic priest is set up as a "Master in Israel"; he has received from Jesus Christ the office and commission of teaching truth: "Teach . . . all nations." He must teach the truth that heals and saves; and because of this teaching, like the Apostle of the Gentiles, he has a duty towards "the learned and the unlearned." But how can he teach unless he himself possess knowledge? "The lips of the priest shall keep knowledge, and they shall seek the law at his mouth," said the Holy Spirit in the Prophecy of Malachy. Who could ever utter a word in praise of sacerdotal learning more weighty than that which divine Wisdom itself once spoke by the mouth of Osee: "Because thou hast rejected knowledge, I will reject thee that thou shalt not do the office of priesthood to Me." The priest should have full grasp of the Catholic teaching on faith and morals; he should know how to present it to others; and he should be able to give the reasons for the dogmas, laws and observances of the Church of which he is minister. Profane sciences have indeed made much progress; but in religious questions there is much ignorance still darkening the mind of our contemporaries. This ignorance the priest must dispel. Never was more pointed than today the warning of Tertullian, " Hoc unum gestit interdum (veritas), ne ignorata damnetur ," "This alone truth sometime craves, that it be not condemned unheard." It is the priest's task to clear away from men's minds the mass of prejudices and misunderstandings which hostile adversaries have piled up; the modern mind is eager for the truth, and the priest should be able to point it out with serene frankness; there are souls still hesitating, distressed by doubts, and the priest should inspire courage and trust, and guide them with calm security to the safe port of faith, faith accepted by both head and heart; error makes its onslaughts, arrogant and persistent, and the priest should know how to meet them with a defense vigorous and active, yet solid and unruffled.
Source: Ad Catholici Sacerdotii (Vatican.va)