18 It is to be anticipated that perhaps not everyone will easily accept this particular teaching. There is too much clamorous outcry against the voice of the Church, and this is intensified by modern means of communication. But it comes as no surprise to the Church that she, no less than her divine Founder, is destined to be a "sign of contradiction." (22) She does not, because of this, evade the duty imposed on her of proclaiming humbly but firmly the entire moral law, both natural and evangelical. Since the Church did not make either of these laws, she cannot be their arbiter—only their guardian and interpreter. It could never be right for her to declare lawful what is in fact unlawful, since that, by its very nature, is always opposed to the true good of man. In preserving intact the whole moral law of marriage, the Church is convinced that she is contributing to the creation of a truly human civilization. She urges man not to betray his personal responsibilities by putting all his faith in technical expedients. In this way she defends the dignity of husband and wife. This course of action shows that the Church, loyal to the example and teaching of the divine Savior, is sincere and unselfish in her regard for men whom she strives to help even now during this earthly pilgrimage "to share God's life as sons of the living God, the Father of all men." (23)
Source: Humanae Vitae (Vatican.va)