20 The teaching of the Church regarding the proper regulation of birth is a promulgation of the law of God Himself. And yet there is no doubt that to many it will appear not merely difficult but even impossible to observe. Now it is true that like all good things which are outstanding for their nobility and for the benefits which they confer on men, so this law demands from individual men and women, from families and from human society, a resolute purpose and great endurance. Indeed it cannot be observed unless God comes to their help with the grace by which the goodwill of men is sustained and strengthened. But to those who consider this matter diligently it will indeed be evident that this endurance enhances man's dignity and confers benefits on human society.
Source: Humanae Vitae (Vatican.va)