4 We have decided therefore to speak to you, Venerable Brethren, and through you to the whole Church of Christ and indeed to the whole human race, on the nature and dignity of Christian marriage, on the advantages and benefits which accrue from it to the family and to human society itself, on the errors contrary to this most important point of the Gospel teaching, on the vices opposed to conjugal union, and lastly on the principal remedies to be applied. In so doing We follow the footsteps of Our predecessor, Leo XIII, of happy memory, whose Encyclical Arcanum , published fifty years ago, We hereby confirm and make Our own, and while We wish to expound more fully certain points called for by the circumstances of our times, nevertheless We declare that, far from being obsolete, it retains its full force at the present day.
Source: Casti Connubii (Vatican.va)