25 And now We turn in a special way to Our own sons and daughters, to those most of all whom God calls to serve Him in the state of marriage. While the Church does indeed hand on to her children the inviolable conditions laid down by God's law, she is also the herald of salvation and through the sacraments she flings wide open the channels of grace through which man is made a new creature responding in charity and true freedom to the design of his Creator and Savior, experiencing too the sweetness of the yoke of Christ. (31) In humble obedience then to her voice, let Christian husbands and wives be mindful of their vocation to the Christian life, a vocation which, deriving from their Baptism, has been confirmed anew and made more explicit by the Sacrament of Matrimony. For by this sacrament they are strengthened and, one might almost say, consecrated to the faithful fulfillment of their duties. Thus will they realize to the full their calling and bear witness as becomes them, to Christ before the world. (32) For the Lord has entrusted to them the task of making visible to men and women the holiness and joy of the law which united inseparably their love for one another and the cooperation they give to God's love, God who is the Author of human life. We have no wish at all to pass over in silence the difficulties, at times very great, which beset the lives of Christian married couples. For them, as indeed for every one of us, "the gate is narrow and the way is hard, that leads to life." (33) Nevertheless it is precisely the hope of that life which, like a brightly burning torch, lights up their journey, as, strong in spirit, they strive to live "sober, upright and godly lives in this world," (34) knowing for sure that "the form of this world is passing away." (35)
Source: Humanae Vitae (Vatican.va)