6 These considerations, Venerable Brethren, move us incessantly to extol andrecommend to Catholic peoples this excellent and most salutary form of devotion.Yet another very urgent reason, of which we have often spoken both in Lettersand Allocutions, encourages us to do this. For that earnest desire, which Wehave learnt from the Divine Heart of Jesus, of fostering the work ofreconciliation among those who are separated from Us daily urges Us morepressingly to action; and we are convinced that this most excellent Re-unioncannot be better prepared and strengthened than by the power of prayer. Theexample of Christ is before us, for in order that His disciples might be one infaith and charity, he poured forth prayer and supplication to His Father. Andconcerning the efficacious prayer of His most holy Mother for the same end,there is a striking testimony in the Acts of the Apostles. Therein is describedthe first assembly of the Disciples, expecting with earnest hope and prayer thepromised fullness of the Holy Spirit. And the presence of Mary united with themin prayer is specially indicated: All these were persevering with one mind inprayer with Mary the Mother of Jesus (Acts i., 14). Wherefore as the nascentchurch rightly joined itself in prayer with her as the patroness and most excellent custodian of Unity, so in these times is it most opportune to do the same all over the Catholic World, particularly during the whole month of October, which we have long ago decreed to be dedicated and consecrated, by the solemn devotion of the Rosary, to the Divine Mother, in order to implore her for the afflicted Church. Let then the zeal for this prayer everywhere be re-kindled, particularly for the end of Holy Unity. Nothing will be more agreeable and acceptable to Mary; for, as she is most closely united with Christ she especially wishes and desires that they who have received the same Baptism with Him may be united with Him and with one another in the same faith and perfect charity. So may the sublime mysteries of this same faith by means of the Rosary devotion be more deeply impressed in men's minds, with the happy result that "we may imitate what they contain and obtain what they promise."
Source: Fidentem Piumque Animum (Vatican.va)