5 Since, therefore, it is clearly evident that this form of prayer is particularly pleasing to the Blessed Virgin, and that it is especially suitable as a means of defence for the Church and all Christians, it is in no way wonderful that several others of Our Predecessors have made it their aim to favour and increase its spread by their high recommendations. Thus Urban IV, testified that "every day the Rosary obtained fresh boon for Christianity." Sixtus IV declared that this method of prayer "redounded to the honour of God and the Blessed Virgin, and was well suited to obviate impending dangers;" Leo X that "it was instituted to oppose pernicious heresiarchs and heresies;" while Julius III called it "the glory of the Church." So also St. Pius V., that "with the spread of this devotion the meditations of the faithful have begun to be more inflamed, their prayers more fervent, and they have suddenly become different men; the darkness of heresy has been dissipated, and the light of Catholic faith has broken forth again." Lastly Gregory XIII in his turn pronounced that "the Rosary had been instituted by St. Dominic to appease the anger of God and to implore the intercession of the Blessed Virgin Mary."
Source: Supremi Apostolatus Officio (Vatican.va)