An excerpt from Our Lady: Queen of the Religious Life, by Fr. Louis Colin C.SS.R.
The impeccability of the Virgin finds its explanation and its origin, first of all, in the Divine goodness, and then in the very plenitude of her interior life. It is the common work of God and Mary.
Our Lady owed the privilege of her moral incorruptibility in the first place to the wisdom, to the power and to the love of the Lord. Does not every perfect gift come from the Father of lights? The Trinity, who had presided at her creation and had kept all stain from her, must thereafter and for the same reasons watch over that first innocence and preserve it from every sin and imperfection. The future Mother of the Word, from the moment of her Immaculate Conception, and by the very merits of her Son, was forever confirmed in grace and in total purity.
"Logically Our Lady's Immaculate Conception brought with it her confirmation in grace; for the reasons that required that she be kept from the shadow of sin and from even the most ephemeral domination of the devil over her at the first moment of her existence, had efficacy and value for every moment of her life" (D. Gaston Desmaret).
Immunity from evil in Mary is explained also by reasons of the psychological order: exemption from all concupiscence, the depth of her faith, the perfection of her virtues and gifts, and especially the plenitude and ardor of her charity. In addition to a very special protection by Providence, Our Lady found in Providence an impregnable rampart and an invincible armor against all which could threaten her moral integrity and tarnish the splendor of her soul. Illumined as she was in her sublime contemplation, on the holiness of God, His sacred and imprescriptible rights, "seeing, as she did, in all sin a monstrosity, an injury against the Most High, a destruction of the order that the love of God has conceived for men" [William, La Vie de Marie, Mère de Jésus, I, 20, 21); free as she was from all passionate impulses and from all enslavement of the senses, with no possible collusion with the world, its spirit and its maxims, bountifully supplied with an incomparable sanctifying grace, rich with heroic virtues, always under the power of the Holy Ghost--how easy it was for the immaculate, "Beati immaculati in via--Blessed are the undefiled in the way" (Ps. 118: 1).
But what, perhaps more than anything else, enlightens and confirms this mystery of the innocence of Mary is the immensity of her love. "Mary ... had no other thought, no other desire, no other joy than God; ... her soul being as it were ... in a continual contemplation of God, the acts of love she made were innumerable.... She passed her life in contemplation ... frequently repeating an act of love" (Saint Alphonsus, The Glories of Mary). It is impossible actually to love God with all one's heart, with all one's strength and at the same time to depart even a little from His will and from His good pleasure. "Divine love inflamed her so that not the slightest imperfection could ever penetrate her life" (Richard of Saint Victor, De Emmanuele, II, XXIX).
The heart of Mary was a "brazier of candor," without smoke or cinder so that the flame which mounted from it was clear and consuming. "Mary is fair, from the root of her conception to the achievement of her glory" (Sertilanges, "Mois de Marie," in Les Cahiers de la Vierge). Conceived as she was in immaculate purity, it is in the same purity that her sanctity must be consummated.
To spot or mar a canvas of Raphael or of Rubens would be an act of vandalism. Our Lady is the purest masterpiece of God; shall we imagine, can we imagine, the Virgin herself defiling and staining that masterpiece?
And not only did Mary keep the brilliance of her original virginity intact, unbesmirched, pure, but that virginity of soul became every instant more and more radiant. She was constantly rising spiritually; the more the Virgin approached the Sun of Justice, the more she was clothed with the sun" (Apoc. 12:1); the more she was united and imbued with God, through faith and love, the more she participated in His infinite purity.
In fine, Our Lady was never more pure, more virginal, more immaculate than at the hour of her death. Her assumption into glory was only the crowning and the triumph of her royal purity.