St Alphonsus Liguori affirms that the Immaculate Conception of Mary means that, full of grace and without stain of sin, Mary was born a Saint.

“To begin with the first point, it is certain that Mary’s soul was the most beautiful that God had ever created: nay more, after the work of the Incarnation of the Eternal Word, this was the greatest and most worthy of himself that an omnipotent God ever did in the world.  St. Peter Damian calls it “a work only surpassed by God” (“Videbis solum Opificem opus istud supergredi”—In Nat. B. V. s. 1).  Hence it follows that divine grace did not come into Mary by drops as in other saints, but like rain on the fleece (“Sicut pluvial in vellus”—Ps. lxxi. 6), as it was foretold by David.  The soul of Mary was like fleece, and imbibed the whole shower of grace, without losing a drop.  St. Basil of Seleucia says, “that the holy Virgin was full of grace, because she was elected and pre-elected by God, and the Holy Spirit was about to take full possession of her” (“Virgo Sancta totam sibi hauserat Spiritus gratiam”—Cat. aur. In Luc. i. 47).  Hence she said, by the lips of Ecclesiasticus, My abode is in the full assembly of saints (“In plenitudine Sanctorum detention mea”—Ecclus. xxiv. 16); that is, as St. Bonaventure explains it, “I hold in plenitude all that other saints have held in part” (“Totum teneo in plenitudine, quod alii Sancti tenent in parte”—De B. V. s. 3).  And St. Vincent Ferrer, speaking particularly of the sanctity of Mary before her birth, says “that the Blessed Virgin was sanctified” (surpassed in sanctity) “in her mother’s womb above all saints and angels” (“Virgi fuit sanctificata super omnes Sanctos et Angelos”—De Nat. B. M. s. 1).

The grace that the Blessed Virgin received exceeded not only that of each particular saint, but of all the angels and saints put together, as the most learned Father Francis Pepe, of the Society of Jesus, proves in his beautiful work on the greatness of Jesus and Mary.  And he asserts that this opinion, so glorious for our Queen, is now generally admitted, and considered as beyond doubt by modern theologians (such as Carthagena, Suarez, Spinelli, Recupito, and Guerra, who have professedly examined the question, and this was never done by the more ancient theologians).  And besides this, he relates that the divine Mother sent Father Martin Guttierez to thank Father Suarez, on her part, for having so courageously defended this most probable opinion, and which, according to Father Segneri, in his “client of Mary,” was afterwards believed and defended by the University of Salamanca.

But if this opinion is general and certain, the other is also very probable; namely, that Mary received this grace, exceeding that of all men and angels together, in the first instance of her Immaculate Conception”

St Alphonsus Liguori – Glories of Mary, Discourse II