24. We wish now to express our encouragement to men of science, who "can considerably advance the welfare of marriage and the family, along with peace of conscience, if by pooling their efforts they labor to explain more thoroughly the various conditions favoring a proper regulation of births. (28) It is particularly desirable that, according to the wish already expressed by Pope Pius XII, medical science succeed in providing a sufficiently secure basis for a regulation of birth, founded on the observance of natural rhythms. (29) In this way, scientists and especially Catholic scientists will contribute to demonstrate in actual fact that, as the Church teaches, "a true contradiction cannot exist between the divine laws pertaining to the transmission of life and those pertaining to the fostering of authentic conjugal love." (30)
 

25. And now our words more directly address our own children, particularly those whom God calls to serve Him in marriage. The Church, while teaching imprescriptible demands of the divine law, announces the tidings of salvation, and by means of the sacraments opens up the paths of grace, which makes man a new creature, capable of corresponding with love and true freedom to the design of his Creator and Savior, and of finding the yoke of Christ to be sweet. (31)

Christian married couples, then, docile to her voice, must remember that their Christian vocation, which began at baptism, is further specified and reinforced by the sacrament of matrimony. By it husband and wife are strengthened and as it were consecrated for the faithful accomplishment of their proper duties, for the carrying out of their proper vocation even to perfection, and the Christian witness which is proper to them before the whole world. (32) To them the Lord entrusts the task of making visible to men the holiness and sweetness of the law which unites the mutual love of husband and wife with their cooperation with the love of God, the author of human life.

We do not at all intend to hide the sometimes serious difficulties inherent in the life of Christian married persons; for them as for everyone else, "the gate is narrow and the way is hard, that leads to life." (33) But the hope of that life must illuminate their way, as with courage they strive to live with wisdom, justice and piety in this present time, (34) knowing that the figure of this world passes away. (35)