And if he will not hear them: tell the church. And if he will not hear the church, let him be to thee as the heathen and publican. Amen I say to you, whatsoever you shall bind upon earth, shall be bound also in heaven; and whatsoever you shall loose upon earth, shall be loosed also in heaven. (Matt 18:17)
No believer will wish to deny that the teaching authority of the Church is competent to interpret even the natural moral law. It is, in fact, indisputable, as our predecessors have many times declared, that Jesus Christ, when communicating to Peter and to the Apostles His divine authority and sending them to teach all nations His commandments, constituted them as guardians and authentic interpreters of all the moral law, not only, that is, of the law of the Gospel, but also of the natural law, which is also an expression of the will of God, the faithful fulfillment of which is equally necessary for salvation…….
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. 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… Given at Rome, 25th day of July, feast of St. James the Apostle, in the year 1968. (Paul VI: Humanae Vitae)
Protesting in front of St. Patricks right after the release of Humanae Vitae