The notion of sanctity, holiness, and saint presents a whole gamut of
meanings. The original and generic meaning of sanctity is that of being
chosen by God and being set apart for himself. This indicates a special
relationship with God and usually endows the person thus privileged with a
particular mission. Baptism is the visible liturgical act by which we are
set apart for God. All baptized can be called saints according to this
understanding of sanctity. In baptism God consecrates us, makes us special
to him and thus holy. There is little doubt that this understanding of
sanctity applies to Mary. She was chosen, commissioned and consecrated as
expressed in the Annunciation scene. The "overshadowing of the Spirit" at
the Annunciation is the ultimate realization of baptism. Mary becomes
God's -- spirit, soul, and body. Being pregnant with- there you have a
very accurate and concrete description of sanctity.
In the Catholic tradition, in particular, sanctity is not a one-way street. Setting us apart, God makes us co-responsible for our consecration. In other words, there is the spontaneous feeling that we ought to live up to our call from God through baptism. We need to prove ourselves worthy of God's grace. This very human reaction of gratitude and love led to a second facet in the understanding of holiness: a virtuous life in conformity with Christ's words and works, and the behavior according to the commandments inherited from the first covenant between Yahweh and his chosen (=holy) people. Concrete forms of this conformity with Christ were, first and foremost, martyrdom for the sake of Christ. The first saints proclaimed and venerated as such were the martyrs. At the time, there was no official process of canonization as we know it today. The well-established witness of somebody's martyrdom and local veneration was sufficient for his/her inclusion into the canon (repertoire) of saints.
Next came the virgins. After the time of persecution, the new ideal of sanctity was found in a life of radical conformity with Jesus Christ. The practical expression is that of the evangelical counsels of chastity, poverty and obedience. Likewise, people of great wisdom and insight into the mysteries of Christianity, the so-called doctors or teachers, qualified for the title of saint. However, up until the tenth century no historically verified canonization by the pope has been reported. The first officially recorded canonization is that of Ulrich of Augsburg by Pope John XV in 993.
All of these aspects- martyrdom, life according to the evangelical
counsels and intimate knowledge of the Christian mysteries- have been
attributed to Mary. The reason thereof is her intimacy with, radical
following and intimate knowledge of Christ. Thus, in this regard, too, the
qualities as saint, although she had never been officially canonized.
Speaking of canonization: The fact that Mary, in particular her vocation story, form part of the canon of Scripture, constitutes the ultimate form of canonization. The holy writers saw in her a consecrated model of Christian existence. Thus, scripture (Mary is the blessed one by God's tangible grace) and tradition (the countless examples of her graceful presence in the life of the Church and its members) are a sure proof of her being a saint even without formal canonization. In fact, she is the foremost of all saints because she was closest to Christ, as mother and disciple and associate.
Calling Mary a saint highlights another important reality. She is a creature enormously graced and privileged by God, but no goddess. Even sinless and Immaculate Conception, she has these privileges because God consecrated her, that is, he set her apart for his service. We may see in her a super-saint but we are not allowed to make her a divinity. This would not only destroy the very concept of sanctity but also make of Mary the impossible ideal or model some people feel she is.