Part 1 from HV 4. 

"4. This kind of question requires from the teaching authority of the Church a new and deeper reflection on the principles of the moral teaching on marriage—a teaching which is based on the natural law as illuminated and enriched by divine Revelation.

No member of the faithful could possibly deny that the Church is competent in her magisterium to interpret the natural moral law. It is in fact indisputable, as Our predecessors have many times declared, (l) that Jesus Christ, when He communicated His divine power to Peter and the other Apostles and sent them to teach all nations His commandments, (2) constituted them as the authentic guardians and interpreters of the whole moral law, not only, that is, of the law of the Gospel but also of the natural law. For the natural law, too, declares the will of God, and its faithful observance is necessary for men's eternal salvation. (3)....."