28 And now, beloved sons, you who are priests, you who in virtue of your sacred office act as counselors and spiritual leaders both of individual men and women and of families—We turn to you filled with great confidence. For it is your principal duty—We are speaking especially to you who teach moral theology—to spell out clearly and completely the Church's teaching on marriage. In the performance of your ministry you must be the first to give an example of that sincere obedience, inward as well as outward, which is due to the magisterium of the Church. For, as you know, the pastors of the Church enjoy a special light of the Holy Spirit in teaching the truth. (39) And this, rather than the arguments they put forward, is why you are bound to such obedience. Nor will it escape you that if men's peace of soul and the unity of the Christian people are to be preserved, then it is of the utmost importance that in moral as well as in dogmatic theology all should obey the magisterium of the Church and should speak as with one voice. Therefore We make Our own the anxious words of the great Apostle Paul and with all Our heart We renew Our appeal to you: "I appeal to you, brethren, by the name of our Lord Jesus Christ, that all of you agree and that there be no dissensions among you, but that you be united in the same mind and the same judgment." (40)
Source: Humanae Vitae (Vatican.va)