The Catechism explains that Mary, the all-holy and ever-virgin mother of God, is “the masterwork of the mission of the Son and the Spirit in the fullness of time” (721). She was a “first” in God’s plan of salvation.
Because the Holy Spirit had prepared her, the Father found Mary a fitting dwelling place where his Son and his Spirit could dwell among human beings. The teaching of the Scripture and the church’s tradition about the abode of wisdom are now understood more fully in relation to Mary. Mary is acclaimed and represented in the liturgy as the Seat of Wisdom (721).
In his role as the Sanctifier, the one who makes people holy, the Holy Spirit was present to fully sanctify Mary at the moment of her own immaculate conception. What happens to Christians at baptism—being cleansed from sin by the living water of the Spirit and being filled with God’s new, abundant life—must have happened to Mary when she was conceived. How else would she have been sanctified as a spotless vessel prepared to bear and bring forth God himself into our world? How else but through the power of the Holy Spirit could Mary have persevered in absolute holiness and dedication to God from the moment of her conception, to her young adulthood (when Jesus was conceived in her), and to the end of her life on earth? Mary is the model of holiness through the work of the Holy Spirit, the Sanctifier, who gave her this unique grace of holiness, freeing her throughout her life from all stain of sin.
It is notable that in the Bible, Gabriel greeted Mary by saying, “Hail, full of grace . . .” before she had said yes to God’s plan and had been overshadowed by the Holy Spirit. Therefore, Mary must have been “full of grace,” and hence filled with the Holy Spirit, even before her “espousal” to the Holy Spirit to bring forth Jesus. You might say that Mary was baptized with the Holy Spirit at the moment of her conception. Then, at the moment she said yes to God, she became both the bride of the Holy Spirit and, through him, the mother of Jesus. Jesus began to dwell within Mary at the moment the Holy Spirit overshadowed her at the annunciation.
This is a selection from Your Life in the Holy Spirit: What Every Catholic Needs to Know and Experience by Dr. Alan Schreck (The Word Among Us, 1995). Available at wau.org/books.