Hail Mary, full of grace,
the Lord is with thee.
Blessed art thou among women
and blessed is the fruit of thy womb, Jesus
Holy Mary, mother of God,
pray for us sinners
now and at the hour of our death.
Luke 1:28 Hail, full of grace! The expression, "full of grace" (kecharitomene), although sometimes translated as "highly favored daughter," represents a proper name and as such expresses a characteristic quality of Mary. The main reason why the pleasure of God rests in a special way on her is her having been chosen the mother of God. Since she is to be the mother of God, and God and sin cannot exist together, she must be free from all sin (including original sin).
Luke 1:42 Elizabeth, filled with the Holy Spirit, says to Mary: "Blessed are you among women, and blessed is the fruit of your womb." In this verse the blessing of God which rests upon Mary is made parallel to the blessing of God which rests upon Jesus in her womb. Again, this parallelism suggests that Mary, just like Jesus, was free from sin from the beginning of her existence.
The title Mother of God was not invented by the Catholic Church, it has Holy Scripture as its basis. If we search all through the first chapter of the Gospel of Luke, we find that the word Lord is used 17 times. In 16 of those cases, it is used synonymously with God. The 17th case is where Elizabeth says to Mary: "Who am I that the mother of my Lord should come to me?" (Luke 1:43). There is no reason to believe that the word Lord in this case is not also used as a synonym for God which means that Holy Scripture has Elizabeth declaring that Mary is the Mother of God. This is a title for Mary which has existed since the time of Christ.
Sometimes Catholics are accused of worshiping Mary as though she were a god. True devotion to Mary HONORS Mary; Catholics WORSHIP God alone. When we pray to Mary, we are praying for her intercession for us. We ask many different people to pray for us - what better person to ask then His own mother. The Church teaches that NO ONE who ever prayed to her was left unaided. The Doctrine of the Communion of Saints teaches us that those who are close to God will intercede on our behalf. The New Testament reveals a valuable lesson concerning Mary: Jesus does answer her pleas for others. Mary's role in salvation is to give her Son to humanity. She leads us to Him, shows us how to live in response to Him, and intercedes on our behalf.