19 Our words would not be an adequate expression of the thought and solicitude of the Church, Mother and Teacher of all peoples, if, after having recalled men to the observance and respect of the divine law regarding matrimony, they did not also support mankind in the honest regulation of birth amid the difficult conditions which today afflict families and peoples. The Church, in fact, cannot act differently toward men than did the Redeemer. She knows their weaknesses, she has compassion on the multitude, she welcomes sinners. But at the same time she cannot do otherwise than teach the law. For it is in fact the law of human life restored to its native truth and guided by the Spirit of God. (24) Observing the Divine Law.
Source: Humanae Vitae (Vatican.va)