109 So, Venerable Brethren, we make entirely Our own the words which Our predecessor of happy memory, Leo XIII, in his encyclical letter on Christian marriage addressed to the bishops of the whole world: "Take care not to spare your efforts and authority in bringing about that among the people committed to your guidance that doctrine may be preserved whole and unadulterated which Christ the Lord and the apostles, the interpreters of the divine will, have handed down, and which the Catholic Church herself has religiously preserved, and commanded to be observed by the faithful of every age."
Source: Casti Connubii (Vatican.va)