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God preserved Mary from the stain of original sin because she was to be the mother of His Son.it follows that He will also have preserved Mary from the stain of any actual sin.
as the goodness of the parents brings honor to their children,,so,too, does the sinfulness of the parents bring shame to their children.But it is NOT fitting that the Mother of God should bring shame to Her divine son.Therefor.God preserved Mary from any personal sin,wether mortal or venial.
Through the Grace of God,which was infused into her soul at the moment of her conception,at the very instant He created her soul and united it to her body. He did it in virtue of the merits of Christ.No descendant of Adam receives the Grace of God except through the merits of Christ.
The Mother of Christ was no exception to this law of Grace.
Like every other human being who is descended of carnal generation from Adam,the blessed virgin Mary need to be redeemed by the blood of Christ,But wheras every other human being needs to be cleansed from the stain of original sin-which has contracted by way of carnal generation from Adam-the virgin Mary did NOT need to be cleansed from original sin.Through the Grace of Christ she was preserved from the stain of sin.
Mary is closer to Christ than any other human being,because He took flesh from her and dwelt in her womb.
The closer one is to Christ,the source of all Grace,the greater degree of Grace one receives from Christ.Mary,therefor,received from Christ a fullness of Grace not granted to any other creature.
Her Immaculate Conception made her worthy to be Mother of God