If God filled us all with grace then where is His Justice? What would be the price paid for Adam’s Original Sin if we all were born full of grace? Why would Jesus have died if we all can be filled with grace as if Adam never sinned?
The Church teaches that the Blessed Mother had this as a singular grace because she was to bear Jesus. Mary’s freedom from Original Sin was an unmerited gift of God and an exception form the law which was vouchsafed to her only.
The Blessed Mother was born without Original Sin because God applied the merits of Christ to her. Because God is outside of time, He can apply the merits of Christ to Mary even if she came before His Birth. So even the Blessed Mother was redeemed by Christ:
[i:21dadpga]491 Through the centuries the Church has become ever more
aware that Mary, “full of grace” through God, was redeemed
from the moment of her conception. That is what the dogma of the
Immaculate Conception confesses, as Pope Pius IX proclaimed in 1854:
The most Blessed Virgin Mary was, from the first moment
of her conception, by a singular grace and privilege of
almighty God and by virtue of the merits of Jesus Christ,
Savior of the human race, preserved immune from all stain
of original sin.
492 The “splendor of an entirely unique holiness” by which
Mary is “enriched from the first instant of her conception” comes
wholly from Christ: she is “redeemed, in a more exalted fashion,
by reason of the merits of her Son.” [/i:21dadpga]
Catechism of the Catholic Church
[i:21dadpga]53. …….. Redeemed by reason of the merits of her Son and united to Him by a close and indissoluble tie,she is endowed with the high office and dignity of being the Mother of the Son of God, by which account she is also the beloved daughter of the Father and the temple of the Holy Spirit. Because of this gift of sublime grace she far surpasses all creatures, both in heaven and on earth. [/i:21dadpga]