Today’s Catechism sections discuss the Immaculate Conception of Mary. No, it was not planned this way! God is great! Anyway, supporting material comes from the Encyclical that declared the Immaculate Conception as an infallible teaching of the Church – “Ineffabilis Deus” by Pope Pius IX and also from “Against Heresies” by St. Irenaeus.

The Immaculate Conception

490 To become the mother of the Savior, Mary "was enriched by God with gifts appropriate to such a role."132 The angel Gabriel at the moment of the annunciation salutes her as "full of grace".133 In fact, in order for Mary to be able to give the free assent of her faith to the announcement of her vocation, it was necessary that she be wholly borne by God's grace.

491 Through the centuries the Church has become ever more aware that Mary, "full of grace" through God,134 was redeemed from the moment of her conception. That is what the dogma of the Immaculate Conception confesses, as Pope Pius IX proclaimed in 1854:

The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Savior of the human race, preserved immune from all stain of original sin.135

492 The "splendor of an entirely unique holiness" by which Mary is "enriched from the first instant of her conception" comes wholly from Christ: she is "redeemed, in a more exalted fashion, by reason of the merits of her Son".136 The Father blessed Mary more than any other created person "in Christ with every spiritual blessing in the heavenly places" and chose her "in Christ before the foundation of the world, to be holy and blameless before him in love".137

493 The Fathers of the Eastern tradition call the Mother of God "the All-Holy" (Panagia), and celebrate her as "free from any stain of sin, as though fashioned by the Holy Spirit and formed as a new creature".138 By the grace of God Mary remained free of every personal sin her whole life long.
"Let it be done to me according to your word. . ."

494 At the announcement that she would give birth to "the Son of the Most High" without knowing man, by the power of the Holy Spirit, Mary responded with the obedience of faith, certain that "with God nothing will be impossible": "Behold, I am the handmaid of the Lord; let it be [done] to me according to your word."139 Thus, giving her consent to God's word, Mary becomes the mother of Jesus. Espousing the divine will for salvation wholeheartedly, without a single sin to restrain her, she gave herself entirely to the person and to the work of her Son; she did so in order to serve the mystery of redemption with him and dependent on him, by God's grace:140

As St. Irenaeus says, "Being obedient she became the cause of salvation for herself and for the whole human race."141 Hence not a few of the early Fathers gladly assert. . .: "The knot of Eve's disobedience was untied by Mary's obedience: what the virgin Eve bound through her disbelief, Mary loosened by her faith."142 Comparing her with Eve, they call Mary "the Mother of the living" and frequently claim: "Death through Eve, life through Mary."143

In his encyclical, “Ineffabilis Deus”, Pope Pius IX declared on December 8, 1854 solemnly and infallibly that Mary was conceived without the stain of original sin.

The Definition

Wherefore, in humility and fasting, we unceasingly offered our private prayers as well as the public prayers of the Church to God the Father through his Son, that he would deign to direct and strengthen our mind by the power of the Holy Spirit. In like manner did we implore the help of the entire heavenly host as we ardently invoked the Paraclete. Accordingly, by the inspiration of the Holy Spirit, for the honor of the Holy and undivided Trinity, for the glory and adornment of the Virgin Mother of God, for the exaltation of the Catholic Faith, and for the furtherance of the Catholic religion, by the authority of Jesus Christ our Lord, of the Blessed Apostles Peter and Paul, and by our own: "We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance of her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Savior of the human race, was preserved free from all stain of original sin, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful."[29]

Hence, if anyone shall dare -- which God forbid! -- to think otherwise than as has been defined by us, let him know and understand that he is condemned by his own judgment; that he has suffered shipwreck in the faith; that he has separated from the unity of the Church; and that, furthermore, by his own action he incurs the penalties established by law if he should are to express in words or writing or by any other outward means the errors he think in his heart.

St. Irenaeus in “Against Heresies” (3, 22, 4) explores the connection between Eve and Mary, who is the “New Eve”:

4. In accordance with this design, Mary the Virgin is found obedient, saying, “Behold the handmaid of the Lord; be it unto me according to your word.” (Luke 1:38) But Eve was disobedient; for she did not obey when as yet she was a virgin. And even as she, having indeed a husband, Adam, but being nevertheless as yet a virgin (for in Paradise they were both naked, and were not ashamed, (Genesis 2:25) inasmuch as they, having been created a short time previously, had no understanding of the procreation of children: for it was necessary that they should first come to adult age, and then multiply from that time onward), having become disobedient, was made the cause of death, both to herself and to the entire human race; so also did Mary, having a man betrothed [to her], and being nevertheless a virgin, by yielding obedience, become the cause of salvation, both to herself and the whole human race. And on this account does the law term a woman betrothed to a man, the wife of him who had betrothed her, although she was as yet a virgin; thus indicating the back-reference from Mary to Eve, because what is joined together could not otherwise be put asunder than by inversion of the process by which these bonds of union had arisen; so that the former ties be cancelled by the latter, that the latter may set the former again at liberty. And it has, in fact, happened that the first compact looses from the second tie, but that the second tie takes the position of the first which has been cancelled. For this reason did the Lord declare that the first should in truth be last, and the last first. (Matthew 19:30), Matthew 20:16 And the prophet, too, indicates the same, saying, instead of fathers, children have been born unto you. For the Lord, having been born the First-begotten of the dead, (Revelation 1:5) and receiving into His bosom the ancient fathers, has regenerated them into the life of God, He having been made Himself the beginning of those that live, as Adam became the beginning of those who die. (1 Corinthians 15:20-22) Wherefore also Luke, commencing the genealogy with the Lord, carried it back to Adam, indicating that it was He who regenerated them into the Gospel of life, and not they Him. And thus also it was that the knot of Eve's disobedience was loosed by the obedience of Mary. For what the virgin Eve had bound fast through unbelief, this did the Virgin Mary set free through faith.

Footnotes

132 LG 56.
133 ⇒ Lk 1:28.
134 ⇒ Lk 1:28.
135 Pius IX, Ineffabilis Deus (1854): DS 2803.
136 LG 53, 56.
137 Cf. ⇒ Eph 1:3-4.
138 LG 56.
139 ⇒ Lk 1:28-38; cf. ⇒ Rom 1:5.
140 Cf. LG 56.
141 St. Irenaeus, Adv. haeres. 3, 22, 4: PG 7/1, 959A.
142 St. Irenaeus, Adv. haeres. 3, 22, 4: PG 7/1, 959A.
143 LC 56; St. Epiphanius, Panarion 2, 78, 18: PG 42, 728CD-729AB; St. Jerome, Ep. 22, 21: PL 22, 408.