16 Now, if we cannot expect to reap a harvest when no seed has been planted, how can we hope to have a people with sound morals if Christian doctrine has not been imparted to them in due time? It follows, too, that if faith languishes in our days, if among large numbers it has almost vanished, the reason is that the duty of catechetical teaching is either fulfilled very superficially or altogether neglected. It will not do to say, in excuse, that faith is a free gift of God bestowed upon each one at Baptism. True enough, when we are baptized in Christ, the habit of faith is given, but this most divine seed, if left entirely to itself, by its own power, so to speak, is not like the mustard seed which "grows up. . . and puts out great branches." Man has the faculty of understanding at his birth, but he also has need of his mother's word to awaken it, as it were, and to make it active. So too, the Christian, born again of water and the Holy Spirit, has faith within him, but he requires the word of the teaching Church to nourish and develop it and to make it bear fruit. Thus wrote the Apostle: "Faith then depends on hearing, and hearing on the word of Christ"; and to show the necessity of instruction, he added, "How are they to hear, if no one preaches?"
Source: Acerbo Nimis (Vatican.va)