Just as there has been development with regard to the understanding of the Faith proclaimed by the Church, JPII says that there has been an analogous development in the Church’s moral teaching.
In her reflection on morality, the Church has always kept in mind the words of Jesus to the rich young man. Indeed, Sacred Scripture remains the living and fruitful source of the Church’s moral doctrine; as the Second Vatican Council recalled, the Gospel is “the source of all saving truth and moral teaching”. The Church has faithfully preserved what the word of God teaches, not only about truths which must be believed but also about moral action, action pleasing to God (cf. 1 Th 4:1); she has achieved a doctrinal development analogous to that which has taken place in the realm of the truths of faith. Assisted by the Holy Spirit who leads her into all the truth (cf. Jn 16:13), the Church has not ceased, nor can she ever cease, to contemplate the “mystery of the Word Incarnate”, in whom “light is shed on the mystery of man”.
29. The Church’s moral reflection, always conducted in the light of Christ, the “Good Teacher”, has also developed in the specific form of the theological science called “moral theology”, a science which accepts and examines Divine Revelation while at the same time responding to the demands of human reason. Moral theology is a reflection concerned with “morality”, with the good and the evil of human acts and of the person who performs them; in this sense it is accessible to all people. But it is also “theology”, inasmuch as it acknowledges that the origin and end of moral action are found in the One who “alone is good” and who, by giving himself to man in Christ, offers him the happiness of divine life. [VS §§28-29, italics in original]
I suppose one example of this development might be seen in the prohibition of polygamy. In contrast to the Old Testament’s regulation of polygamous relationships (see for example Dt 21:15-17), the New Testament says that the Church’s married leaders must be monogamous. It seems reasonable to conclude that over time the Church banned polygamy completely as a consequence of reflection upon this requirement and the teaching of Scripture in Gn 2:24 and Mt 19:1-12. It would be unreasonable to suppose that men grow in understanding of creation, and that the Church’s understanding of the truths of Faith advances, but that Her understanding of the way a man should live has not, or does not, or cannot develop.
Unfortunately, just as heresy often motivates doctrinal development, so too moral errors at least sometimes require development of the Church’s teaching as well. So the Pope writes:
The work of many theologians who found support in the Council’s encouragement has already borne fruit in interesting and helpful reflections about the truths of faith to be believed and applied in life, reflections offered in a form better suited to the sensitivities and questions of our contemporaries. The Church, and particularly the Bishops, to whom Jesus Christ primarily entrusted the ministry of teaching, are deeply appreciative of this work, and encourage theologians to continue their efforts, inspired by that profound and authentic “fear of the Lord, which is the beginning of wisdom” (cf. Prov 1:7).
At the same time, however, within the context of the theological debates which followed the Council, there have developed certain interpretations of Christian morality which are not consistent with “sound teaching” (2 Tim 4:3). Certainly the Church’s Magisterium does not intend to impose upon the faithful any particular theological system, still less a philosophical one. Nevertheless, in order to “reverently preserve and faithfully expound” the word of God,48 the Magisterium has the duty to state that some trends of theological thinking and certain philosophical affirmations are incompatible with revealed truth. [VS §29, emphasis in original]
It is a primary purpose of the encyclical to address certain errors of moral theology.