From Pope St. John Paul II’s Apostolic Exhortation Familiaris Consortio (1981):
With the creation of man and woman in His own image and likeness, God crowns and brings to perfection the work of His hands: He calls them to a special sharing in His love and in His power as Creator and Father, through their free and responsible cooperation in transmitting the gift of human life: “God blessed them, and God said to them, ‘Be fruitful and multiply, and fill the earth and subdue it.'”80
Thus the fundamental task of the family is to serve life, to actualize in history the original blessing of the Creator—that of transmitting by procreation the divine image from person to person.81
Fecundity is the fruit and the sign of conjugal love, the living testimony of the full reciprocal self-giving of the spouses: “While not making the other purposes of matrimony of less account, the true practice of conjugal love, and the whole meaning of the family life which results from it, have this aim: that the couple be ready with stout hearts to cooperate with the love of the Creator and the Savior, who through them will enlarge and enrich His own family day by day.”82
However, the fruitfulness of conjugal love is not restricted solely to the procreation of children, even understood in its specifically human dimension: it is enlarged and enriched by all those fruits of moral, spiritual and supernatural life which the father and mother are called to hand on to their children, and through the children to the Church and to the world.
80 Gn 1:28.
81 Cf. Gn 5:1-3.
82 Pastoral Constitution on the Church in the Modern World Gaudium et spes, 50.
— Familiaris Consortio 28