13 And again, if we consider the matter with attention, and especially if we consider the burning and sweet love which Almighty God without doubt had, and has, for the mother of His only begotten Son, for what reason can we even think that she was, even for the briefest moment of time, subject to sin and destitute of divine grace. Almighty God could certainly, by virtue of the merits of the Redeemer, bestow on her this singular privilege; that therefore He did not do so, we cannot even suppose. It was fitting that Jesus Christ should have such a mother as would be worthy of Him as far as possible; and she would not have been worthy, if, contaminated by the hereditary stain even for the first moment only of her conception, she had been subject to the abominable power of Satan.
Source: Fulgens Corona (Vatican.va)