
The Catechism, quoting the Byzantine Liturgy, states, "The Assumption of the Blessed Virgin is a singular participation in her Son's resurrection and an anticipation of the resurrection of other Christians: 'In giving birth you kept your virginity; in your Dormition you did not leave the world, O Mother of God, but were joined to the source of Life. You conceived the living God and, by your prayers, will deliver our souls from death'" (No. 966).
We firmly believe that from the first moment of her conception, Mary was free of all sin including original sin by a special favor of Almighty God. The Archangel Gabriel recognized her as "full of grace," "blessed among women," and "one with the Lord." Mary had been chosen to be the mother of our Savior. By the power of the Holy Spirit, she conceived our Lord, Jesus Christ, and through her, true God became also true man: "The Word became flesh and dwelt among us." During her lifetime, although the Gospel citations are limited, Mary always presented our Lord to others: to Elizabeth and her son, John the Baptist, who leapt for joy in the womb at the presence of the Lord still in His own mother's womb; to the simple shepherds as well as the wise Magi; and to the people at Cana when our Lord acquiesced to His mother's wish and performed the first miracle. Mary also stood at the foot of the cross with her Son, supporting Him and sharing in His suffering through her love as only a mother could do; moreover, as the exemplary disciple, she stood there courageously with the hope of the resurrection. Finally, she was with the apostles at Pentecost, when the Holy Spirit descended and the Church was born. Therefore, each of us can step back and see Mary as the faithful servant of God who shared intimately in the birth, life, death, and resurrection of our Lord.
For these reasons, we believe that the promises our Lord has given to each of us of sharing eternal life, including a resurrection of the body, were fulfilled in Mary. Since Mary was free of original sin and its effects (one of which is corruption of the body at death), since she shared intimately in the life of the Lord and in His passion, death and resurrection, and since she was present at Pentecost, this model disciple appropriately shared in the bodily resurrection and glorification of the Lord at the end of her life.
( taken from Catholic Education Resource Center ~~ Father William Saunders )