In the task of transmitting life, therefore, they are not free to proceed completely at will, as if they could determine in a wholly autonomous way the honest path to follow; but they must conform their activity to the creative intention of God, expressed in the very nature of marriage and of its acts, and manifested by the constant teaching of the Church. (Papal Encyclical, Humanae Vitae, 1968).
2. We must solemnly proclaim that human life is transmitted by means of the family, and the family is based upon a marriage which is one and indissoluble and, with respect to Christians, raised to the dignity of a sacrament. The transmission of human life is the result of a personal and conscious act, and, as such, is subject to the all-holy, inviolable and immutable laws of God, which no man may ignore or disobey. He is not therefore permitted to use certain ways and means which are allowable in the propagation of plant and animal life. Genesis relates how God gave two commandments to our first parents: to transmit human life -- "Increase and multiply" -- and to bring nature into their service -- "Fill the earth, and subdue it." These two commandments are complementary. Nothing is said in the second of these commandments about destroying nature. On the contrary, it must be brought into the service of human life. We are sick at heart, therefore, when We observe the contradiction which has beguiled so much modern thinking. On the one hand we are shown the fearful specter of want and misery which threatens to extinguish human life, and on the other hand we find scientific discoveries, technical inventions and economic resources being used to provide terrible instruments of ruin and death. A provident God grants sufficient means to the human race to find a dignified solution to the problems attendant upon the transmission of human life. But these problems can become difficult of solution, or even insoluble, if man, led astray in mind and perverted in will, turns to such means as are opposed to right reason, and seeks ends that are contrary to his social nature and the intentions of Providence. (Papal Encyclical, Mater et Magistra, 15 May 1961).
3. Parents should regard as their proper mission the task of transmitting human life and educating those to whom it has been transmitted. Let them thoughtfully take into account both their own welfare and that of their children, those already born and those which the future may bring. For this accounting they need to reckon with both the material and the spiritual conditions of the times as well as of their state in life. (Second Vatican Council. Gaudium et Spes, 1965).