The birth of the Blessed Virgin Mary announced joy and the near approach of salvation to the lost world; therefore is this festival celebrated by the church with praise and thanksgiving. It was a mystery of sanctity, and distinguished by singular privileges. Mary was brought forth into the world, not like other children of Adam, infected with the loathsome contagion of sin,but pure, holy, beautiful, and glorious, adorned with all the most precious graces which became her who was chosen to be the Mother of God. She appeared indeed in the weak state of our mortality; but in the eyes of heaven she already transcended the highest seraph in purity, brightness, and the richest ornaments of grace. I am black, but beautiful, O ye daughters of Jerusalem. (Cant. 1:4) The spouse says to her much more emphatically than to other souls sanctified by his choicest graces: As the lily among thorns, so is my beloved among the daughters.(Cant. 2:2) Thou art all fair, and there is not a spot in thee.(Cant. 4:7) Man was no sooner fallen in paradise through the woman seduced by the infernal spirit, but God promised another woman whose seed should crush that serpent’s head. I will put enmities, said he to the serpent, between thee and the woman, and thy seed and her seed: she shalt crush thy head, and thou shalt lie in wait for her heel. (Gen. 3:15)

To understand the great present that in her God bestowed on the world, we must consider her transcendent dignity, and the singular privileges by which she was distinguished above all other pure creatures. Her dignity is expressed by the evangelist when he says, That of her was born Jesus, who is called the Christ. From this text alone is that article of the Catholic faith sufficiently evinced, that she is truly Mother of God. It is clear this is not to be understood as if she could be in any sense mother of the Divinity, the very thought whereof would imply contradiction and blasphemy, but by reason that she conceived and brought forth that Blessed Man who subsisting by the second divine person of the adorable Trinity, is consequently the natural, not the adoptive Son of God, which was the Semi-Nestorian error broached by Felix and Elipandus. In the Incarnation the human nature of Christ was assumed by, and hypostatically, that is, intimately and substantially, united to the person of God the Son, so that the actions done by this nature, are the actions of that Divine Person, whose assumed or appropriated nature this is. Hence we truly say with St. Paul, that we are redeemed by the blood of a God, and with the church, that God was born of the Virgin Mary, suffered and died on the cross; all which he did in that human nature which he had wonderfully taken upon him.

...The dignity of Mother of God is the highest to which any mere creature is capable of being raised. What closer alliance could any pure creature have with the Creator of all things? What name could be more noble, what prerogative more singular, or more wonderful? He who was born of the Father from all eternity, the only-begotten and consubstantial Son, Maker and Lord of all things, is born in time, and receives a being in his nature of man from Mary. “Listen and attend, O man,” cries out St. Anselm, “and be transported in an ecstacy of astonishment, contemplating this prodigy. The infinite God had one only-begotten co-eternal Son; yet he would not suffer him to remain only his own, but would also have him to be made the only Son of Mary.” And St. Bernard says: “Choose which you will most admire, the most beneficent condescension of the Son, or the sublime dignity of the Mother. On each side it is a subject of wonder and astonishment; that a God should obey a woman is a humility beyond example, and that a woman commands a God, is a preeminence without a rival.” The first, which is the humiliation of him who is infinite, in itself can bear no comparison with the other; but the astonishing exaltation of Mary transcends what we could have imagined any creature capable of. No creature can he raised to what is infinite: yet the object or term of this dignity of Mary is infinite, and the dignity has a nearer and closer relation to that object than could have been imagined possible by creatures, had not omnipotence made it real. To this transcendent dignity all graces and privileges, how great and singular soever, seem in some measure due. We admire her sanctity, her privileged virginity, all the graces with which she was adorned, and the crown with which she is exalted in glory above the cherubim; but our astonishment ceases when we reflect that she is the Mother of God. In this is every thing great and good, that can suit a mere human creature, naturally comprised.

To take a review of some other singular privileges of this glorious creature, we must further consider that she is both a mother and a spotless virgin. This is the wonderful prerogative of Mary alone; a privilege and honor reserved to her, which shall not be given to any other says St. Bernard. The ancient prophets spoke of it as the distinguishing mark of the Mother of the Messiah and the world’s Redeemer, and frequently call the Christ Jehovah or the true God, as Dr. Waterland demonstrates by many passages. This was the miraculous token of the assured deliverance of mankind by the long expected Saviour, which God himself was to give to the incredulous king Achaz, doubtful and anxious about his present deliverance from his temporal enemies. The Lord himself shall give you a sign, said Isaias: Behold, a Virgin shall conceive, and bear a son, and hisniame shall be called Emmanuel. (Is. 7:14) This must evidently be understood of the Messiah, to whom alone many qualities and epithets in this and the following chapter can agree, though a son of the prophet mentioned afterward was also a present type of the king’s temporal deliverance. The title of Virgin must here mean one who remained such when a mother; for this circumstance is mentioned as a stupendous miracle. Jeremy also, contemplating this mystery in spirit, expressed his astonishment at this prodigy unheard of on earth, that a woman should encompass in her womb a man, the great Redeemer of the world.

...Her virginity was not only a miraculous privilege, but also a voluntary virtue, she having, by an early vow, consecrated her chastity to God, as the fathers infer from her answer to the angel. Such a privileged mother became the Son of God. The earth, defiled by the abominations of impurity was loaded with the curses of God, who said: .My spirit shall not remain in man for ever, because he is flesh. (Gen. 6) But God choosing Mary to take himself flesh of, prepared her for that dignity by her spotless virginity, and on account of that virtue said to her: The Holy Ghost shall come upon thee, and the power of the Most High shall overshadow thee. It is by imitating her perfect purity according to our state, that we shall recommend ourselves to our heavenly spouse, who is the lover of chaste souls, and is called by St. Gregory Nazianzen, the virgin by excellence, and the first of virgins. In the example and patronage of Mary we have a powerful succor against the opposite most abominable and destroying vice. We can only be victorious in its most dangerous conflicts by arming ourselves with her sincere humility, perfect distrust in ourselves, constant spirit of prayer, and flight of the shadow of danger, and with the mortification of our own will, and of our senses and flesh.

The Virgin Mary was the most perfect model of all other virtues. St. Ambrose, in the beginning of his second book, On Virginity, exhorts virgins in particular to make her life the rule of their conduct: “Let the life and virginity of Mary,” says he, “be set before you as in a looking-glass, in which is seen the pattern of chastity and virtue. The first spur to imitation is the nobility of the master. What more noble than the Mother of God !—she was a virgin in body and mind, whose candor was incapable of deceit or disguise; humble in heart ; grave in words; wise in her resolutions. She spoke seldom and little; read assiduously, and placed her confidence, not in inconstant riches, but in the prayers of the poor. Being always employed with fervor, she would have no other witness of her heart but God alone, to whom she referred herself, and all things she did or possessed. She injured no one, was beneficent to all, honored her superiors, envied not equals, shunned vain-glory, followed reason, ardently loved virtue. Her looks were sweet, her discourse mild, her behavior modest. Her actions had nothing unbecoming, her gait nothing of levity, her voice nothing of overbearing assurance. Her exterior was all so well regulated, that in her body was seen a picture of her mind, and an model of all virtues. Her charities knew no bounds; temperate in her diet, she prolonged her fasts several days, and the most ordinary meats were her choice, not to please the taste, but to support nature. The moments which we pass in sleep, were to her a time for the sweetest exercises of devotion. It was not her custom to go out of doors, except to the temple, and this always in the company of her relations,” &c. The humble and perfect virtue of Mary raised in St. Joseph the highest opinion of her sanctity, as appeared when he saw her with child. “This is a testimony of the sanctity of Mary,” says St. Jerom, “that Joseph knowing her chastity, and admiring what had happened, suppresses in silence a mystery which he did not understand.” Another ancient writer improves the same remark, crying out: O inestimable commendation of Mary ! Joseph rather believed her virtue than her womb, and grace rather than nature. He thought it more possible that Mary should have conceived by miracle without a man, that she should have sinned.”

Yet this sanctity of Mary, which was a subject of admiration to the highest heavenly spirits consisted chiefly in ordinary actions, and in the purity of heart and the fervor with which she performed them. All her glory is from within! (Ps. 44:14) From her we learn that our spiritual perfection is to be sought in our own state, and depends very much upon the manner in which we perform our ordinary actions. True virtue loves to do all things in silence and with as little show and noise as may be; it studies to avoid whatever would recommend it to the eyes of men, desiring to have no other witness but him who is its rewarder, and whose glory alone it seeks. A virtue which wants a trumpet to proclaim it, or which affects only public, singular, or extraordinary actions, is to be suspected of subtle pride, vanity, and self-love.

To study these lessons in the life of Mary, to praise God for the graces which he has conferred upon her, and the blessings which through her he has bestowed on the world, and to recommend our necessities to so powerful an advocate, we celebrate festivals in her honor. This of her Nativity has been kept in the church with great solemnity above a thousand years. The Roman Order mentions the homilies and litany which were appointed by pope Sergius in 688 to be read upon it ; and a procession is ordered to he made on this day from St. Adrian’s church to the Liberian basilic or St. Mary Major. In the Sacramentary of St. Gregory the Great, published by Dom Menard, particular collects or prayers are prescribed for the mass, procession, and matins on the Nativity of the Blessed Virgin Mary, with a special preface for the mass. A mass with particular collects for this festival occurs in the old Roman Sacramentary or Missal, published by cardinal Thomasius, which is judged by the learned to be the same that was used by pope Leo the Great, and some of his predecessors. This feast is mentioned by St. Ildefonsus, in the seventh century. The Greeks, the Copths in Egypt, and the other Christian churches in the East, keep with great solemnity the Nativity of the Blessed Virgin. St. Peter Damian pathetically exhorts all the faithful to celebrate it with great devotion.
(edited from Butler's Lives of the Saints)