10 If these praises of the Blessed Virgin Mary be given the careful consideration they deserve, who will dare to doubt that she, who was purer than the angels and at all times pure, was at any moment, even for the briefest instant, not free from every stain of sin? Deservedly, therefore, St. Ephrem addresses her Divine Son in these words: "Really and truly Thou and Thy Mother are alone entirely beautiful. Neither in Thee nor in Thy Mother is there any stain." ( Carmine Nisibena , Ed. Bickell, 123). From these words, it is clearly apparent that there is only one among all holy men and women about whom it can be said that the question of sin does not even arise, and also that she obtained this singular privilege, never granted to anyone else, because she was raised to the dignity of Mother of God.
Source: Fulgens Corona (Vatican.va)