23 Liberation or immunity from this disease of heresy is possible only when the clergy are properly instructed, since "faith. . . depends on hearing, and hearing on the word of Christ." Today we must heed the words of truth. We see this poison penetrating through all the veins of the State (from sources where it would be the least expected) to such an extent that the causes are the same as those Charles records in the following words: "If those who associate with heretics are not firmly rooted in the Faith there is reason to fear that they will easily be seduced by the heretics into the trap of impiety and false doctrine." Nowadays facility in travel and communication has proven just as advantageous for error as for other things. We are living in a perverse society of unbridled license of passions in which "there is no truth...and there is no knowledge of God," in "all the land made desolate, because there is none that considereth in the heart." For that reason, borrowing the words of Charles, "we have already emphasized the importance of having all the faithful of Christ well instructed in the rudiments of Christian doctrine" and have written a special encyclical letter on that extremely important subject. However, We do not wish to repeat the lamentation Borromeo was moved to utter because of his burning zeal, namely, that "up until now We have received very little success in a matter of such importance." Rather, moved like him "by the enormity and danger of the task," We would once again urge everyone to make Charles his model of zeal so that he will contribute in this work of Christian restoration according to his position and ability. Fathers and employers should recall how the holy Bishop frequently and fervently taught that they should not only afford the opportunity but even consider it their duty to see that their children, servants, and employees study Christian doctrine. Clerics should remember that they must assist the parish priests in the teaching of Christian doctrine. Parish priests should erect as many schools for this same purpose as the number and needs of the people demand. They should further take care that they have upright teachers, who will be assisted by men and women of good morals according to the manner the holy Archbishop Milan prescribed.
Source: Editae Saepe (Vatican.va)