18 If anyone desires a confirmation of this it may easily be found in the dogma of the Immaculate Conception of Mary. For leaving aside tradition which, as well as Scripture, is a source of truth, how has this persuasion of the Immaculate Conception of the Virgin appeared so conformed to the Catholic mind and feeling that it has been held as being one, and as it were inborn in the soul of the faithful? "We shrink from saying," is the answer of Dionysius of Chartreux, "of this woman who was to crush the head of the serpent that had been crushed by him and that Mother of God that she had ever been a daughter of the Evil One" ( Sent . d. 3, q. 1). No, to the Christian intelligence the idea is unthinkable that the flesh of Christ, holy, stainless, innocent, was formed in the womb of Mary of a flesh which had ever, if only for the briefest moment, contracted any stain. And why so, but because an infinite opposition separates God from sin? There certainly we have the origin of the conviction common to all Christians that Jesus Christ before, clothed in human nature, He cleansed us from our sins in His blood, accorded Mary the grace and special privilege of being preserved and exempted, from the first moment of her conception, from all stain of original sin.
Source: Ad Diem Illum Laetissimum (Vatican.va)