A papal encyclical is a highly authoritative medium for conveying teachings of the Catholic Church and the papal magesterium. By its nature it represents Catholic doctrinal teaching, and Casti Connubii contains three principal Christian themes, each presented in response to what Pope Pius XI saw as a modern threat to the sanctity of matrimony. All three themes have a rich biblical and Catholic history. The first is to affirm, against a purely secular and sociological view, that marriage is indeed instituted by God. Therefore, marriage is made up of divine and immutable rules that no society or individual can break. These include the indissolubility of marriage, its ordering to the procreation of children, and its relative autonomy and freedom from the interference of the state. The state is obliged to carry out its governing functions while respecting the essentially sacral nature of marriage.
The second theme is that marriage is truly a sacrament among the Christian faithful. This reaffirms a long-standing Catholic belief as opposed to numerous Protestant denominations that do not count marriage as a sacrament or Christian ordinance. As a Christian sacrament, it is subject to the higher law of Jesus Christ. It demands an absolute indissolubility when consummated by baptized spouses. It demands chastity and purity by its partners—forbidding not only actual adultery but also impure thoughts and intentions as well.
The third Christian theme is to require spouses to practice chastity according to their state in life. Because the Anglican Lambeth Conference had just given cautious approval to contraceptive use by married couples, Pius explicitly addresses the question of birth control. He cites “uninterrupted Christian tradition” that forbids the deliberate interference with the generation of new life while explicitly permitting couples to make reasonable use of the natural periods of infertility. Pius presents this then as a full, balanced, and noble portrayal of Christian marriage, rejecting unchaste practices while fostering the noble and intimate union of the spouses.