First of all, the Mother of God was not exempt from the common fallenness of our nature brought about by Adam's sin, and therefore she too had to die.  This true teaching of the Orthodox Church corrects the heresy promulgated in the 19th century by Pope Pius IX, which is called the dogma of the "Immaculate Conception."  This is the teaching that the Mother of God, when she was conceived in the womb of St. Anna, was exempted from the consequences of the sin of Adam.  Later, in 1951, Pius XII proclaimed the dogma of the Assumption of the Mother of God.  Because Pius XII understood the implications of the Immaculate Conception, he could not clearly state that the Holy Virgin actually died, since that would imply that she partook of the Ancestral Sin.  Therefore, he couched his proclamation in language so vague that it is not clear whether the Roman Catholic Church believes that the Holy Virgin actually died or not. 

            The Orthodox Church teaches what the Church has always believed since the Apostolic generation - that the Holy Virgin fully possessed our fallen nature, including the damage caused by sin.  It was precisely this fallen nature which Our Lord united hypostatically (i.e., in His One Divine  Person) to His Divinity when He became incarnate in the Virgin's womb, and thus the entire economy of His kenosis (self-emptying) - His Incarnation, Passion, Death, Resurrection, and Ascension - saved our very own fallen nature and not some other nature.  The Most Holy Theotokos possessed this same human nature made mortal by Adam's sin, and therefore she had to die and to be saved by the Resurrected Christ, just as we all do. 

            At the hour of her holy repose, the Holy Apostles were gathered from the ends of the earth to be with her.  They beheld Her Divine Son come and take her soul gently from her body.  Since her soul left her body, she really and truly died.  Then three days later, they went to her tomb and found it empty.  She appeared to them in heavenly glory and revealed that she had been resurrected before the Common Resurrection and translated to heaven in virtue of her surpassing holiness and the love borne her by her Son. 

            Thus the Orthodox Church teaches the two-fold mystery of the passing of the Most Holy Theotokos:  Her  koimisis (falling asleep - death) and her  metastasis (bodily translation - resurrection and ascension into heaven).  On the one hand, because the Holy Virgin was a fallen human being like you and me, she had to die. On the other hand, because she attained a level of holiness higher than all creation, being more honorable than the Cherubim and more glorious than the Seraphim, death could not hold her pure body, which was resurrected before the general resurrection by her Son.  This is the Final Mystery of the Holy Virgin, the crown of her life of total, unimaginably pure service to the Lord.  It is also the joy of every Orthodox Christian, for each of us looks to her as our own Mother, Protectress, Shelter, Sweetness, and Consolation in this life of many sorrows which would be so unbearable without her motherly tenderness to uphold us and bring us, despite our many sins, to the Throne of her Divine Son.

The Queen Stood at Thy Right Hand          

         Not long after the Theotokos's dormition, it became customary in many places to commemorate her in the Church's worship and liturgy, by her name and even by her titles and office. She was honored in literature and liturgy by Apostolic Fathers in the first century. By the early fourth century, she was the subject of a growing body of Church literature. And from the fifth century forward, virtually all major Church fathers wrote homilies and commentaries about her. From the early fifth century, a general Feast in honor of the Mother of God was celebrated on August 15th in a church near Jerusalem . In the East, it was customary from the late sixth century to observe the Feast of the Annunciation by chanting the Akathist Hymn by St. Romanos the Melodist. In the early sixth century, a Feast dedicated exclusively to the Dormition (Koimisis) of the Theotokos was being celebrated on August 15th at the basilica of Gethsemane in Jerusalem , which also contained her tomb. This Feast spread to near and distant places, and at the end of the sixth century, the Emperor Maurice decreed that the Feast of the Dormition of our Lady the Theotokos was to be observed throughout the Roman Empire . 

            From Mary: the Untrodden Portal of God, by George Gabriel