Assumption (Dormition) Declared de Fide by Pope Pius XII  in 1950.  This is one of the most ancient of doctrines concerning the BVM, though it was the last to be solemnly declared.  This doctrine has been held as true since early days of the Church.  Catholic Church has relics of nearly any saint, yet there are none of Mary. 

St. John Damascene: "It was fitting that she, who had kept her virginity intact in childbirth, should keep her own body free from all corruption even after death. It was fitting that she, who had carried the Creator as a child at her breast, should dwell in the divine tabernacles. It was fitting that the spouse, whom the Father had taken to himself, should live in the divine mansions. It was fitting that she, who had seen her Son upon the cross and who had thereby received into her heart the sword of sorrow which she had escaped in the act of giving birth to him, should look upon him as he sits with the Father. It was fitting that God's Mother should possess what belongs to her Son, and that she should be honored by every creature as the Mother and as the handmaid of God." 

St. Bonaventure "From this we can see that she is there bodily...her blessedness would not have been complete unless she were there as a person. The soul is not a person, but the soul, joined to the body, is a person. It is manifest that she is there in soul and in body. Otherwise she would not possess her complete beatitude.”

The deepening of faith in the virginal motherhood led the Church to confess Mary's real and perpetual virginity even in the act of giving birth to the Son of God made man. LG 52 In fact, Christ's birth "did not diminish his mother's virginal integrity but sanctified it."  And so the liturgy of the Church celebrates Mary as Aeiparthenos, the "Ever-virgin". LG 57 CCC 499

Against the doctrine of the perpetual virginity of Mary is the objection raised that the Bible mentions brothers and sisters of Jesus. The Church has always understood these passages as not referring to other children of the Virgin Mary. In fact James and Joseph, "brothers of Jesus", are the sons of another Mary, a disciple of Christ, whom St. Matthew significantly calls "the other Mary". They are close relations of Jesus, according to an Old Testament expression.

At once virgin and mother, Mary is the symbol and the most perfect realization of the Church: “the Church indeed . . . by receiving the word of God in faith becomes herself a mother. By preaching and Baptism she brings forth sons, who are conceived by the Holy Spirit and born of God, to a new and immortal life. She herself is a virgin, who keeps in its entirely and purity the faith she pledged to her spouse.” CCC 507

Mary "remained a virgin in conceiving her Son, a virgin in giving birth to him, a virgin in carrying him, a virgin in nursing him at her breast, always a virgin" (St. Augustine, Serm. 186, 1: PL 38, 999): with her whole being she is "the handmaid of the Lord" (Lk 1:38).

Council of Ephesus 431

                                                              i.      Battle against heresy of Nestorianism, attributed to Nestorius of Antioch, patriarch of Constantinople: the claim that Mary was not Mother of God only of Jesus Christ, thus, giving her a title of Kristotokos (Christ bearer).  She was only mother of his human nature.  This called into question the unity of the person of Jesus Christ having two natures: divine and human.  Nestorius proclaimed two persons because there were two natures, human and divine.  Cyril of Alexandria defended the teaching of one person and two natures.  “It was not that an ordinary man was born first of the holy Virign, on whom afterwards the WORD descended; what we say is that, being united with the flesh from the womb, the WORD has undergone birth in the flesh, making the birth in the flesh his own, thus, the holy Fathers have unhesitatingly called the holy Virgin “Mother of God” theotokos.” Cyril of Alexandria 430/1

Declared de Fide by Pope Pius IX 1854.  

The real question was one of Original Sin. The Immaculate Conception was debated for centuries prior to the declaration of Pope Pius IX of 1854. In the Constitution Ineffabilis Deus of 8 December, 1854, Pius IX pronounced and defined that the Blessed Virgin Mary "in the first instance of her conception, by a singular privilege and grace granted by God, in view of the merits of Jesus Christ, the Savior of the human race, was preserved exempt from all stain of original sin." The subject of this immunity from original sin is the person of Mary at the moment of the creation of her soul and its infusion into her body.  Numerous Church Fathers believed in the immaculate purity of Mary. 

“Had Mary not been preserved from Original Sin from the very moment she existed, she would have been literally torn apart on having in her womb the incarnate, infinite holiness.  Not only sin, but also mere inclination to sin, are utterly incompatible with the most holy God. This the reason, God’s infinite incarnate holiness, why Mary had to be utterly free from Original Sin.  Anyone with a genuine faith in the holiness of God, can sense this, however implicitly.  This is why the sensus fidelium instinctively embraced that dogma long before it became defined, while the learned theologians kept debating it.”  In the womb, “God, the most holy God, has to be in a total symbiosis not only with the human nature it assumes but also forces that nature to be in a total symbiosis with a woman’s womb as the very matrix of its coming about, of its gradual growth and of its eventual birth.  Such a womb must be immaculate.”  Stanley Jaki OSB 

To you we flee for shelter and compassion, mother of God.

You alone are chaste and blessed;

do not disregard our prayers in this hour of need,

but deliver us from danger.

Early 3^rd Century Prayer