This prayer for the Nativity of the Blessed Virgin Mary refers to Mary being "preserved from original sin," reminding us of her Immaculate Conception. Mary was conceived in the womb of Saint Anne on December 8, the Feast of the Immaculate Conception, which is exactly nine months before September 8, the Feast of the Nativity of the Blessed Virgin Mary. (For more on the reason why Catholics celebrate the birth of Mary, see When Is the Virgin Mary's Birthday?)
The focus of this prayer is on Christian unity. Recalling those who, "although separated from the Church, have kept a certain veneration for" the Blessed Virgin Mary (that is, the Eastern and Oriental Orthodox), the prayer asks the Mother of God to intercede "to restore unity and peace once more to all the Christian people."
The phrase "O Virgin who destroyest all heresies" is ancient, going back to the middle of the first millennium of Christianity, and refers to Mary's role in salvation history. It was Mary's fiat—her acceptance of God's Will—that brought Christ into the world.
This prayer is perfect for a novena in preparation for the Feast of the Nativity of the Blessed Virgin Mary, as well as throughout the year, when praying for Christian unity.
Prayer for the Nativity of the Blessed Virgin Mary
O Virgin immaculate, thou who by a singular privilege of grace wast preserved from original sin, look in pity upon our separated brethren, who are nevertheless thy children, and call them back to the center of unity. Not a few of them, although separated from the Church, have kept a certain veneration for thee; and do thou, generous as thou art, reward them for it, by obtaining for them the grace of conversion.
Thou wast conqueror of the infernal serpent from the first instant of thy existence; renew even now, for it is now more necessary than ever before, thine ancient triumphs; glorify thy divine Son, bring back to Him the sheep that have strayed from the one fold and place them once more under the guidance of the universal Shepherd who holds the place of thy Son on earth; let it be thy glory, O Virgin who destroyest all heresies, to restore unity and peace once more to all the Christian people.