22 But lest it be thought that We have lost sight of Our subject, which is the Immaculate Conception, what great and effectual succour will be found in it for the preservation and right development of those same virtues. What truly is the point of departure of the enemies of religion for the sowing of the great and serious errors by which the faith of so many is shaken? They begin by denying that man has fallen by sin and been cast down from his former position. Hence they regard as mere fables original sin and the evils that were its consequence. Humanity vitiated in its source vitiated in its turn the whole race of man; and thus was evil introduced amongst men and the necessity for a Redeemer involved. All this rejected it is easy to understand that no place is left for Christ, for the Church, for grace or for anything that is above and beyond nature; in one word the whole edifice of faith is shaken from top to bottom. But let people believe and confess that the Virgin Mary has been from the first moment of her conception preserved from all stain; and it is straightway necessary that they should admit both original sin and the rehabilitation of the human race by Jesus Christ, the Gospel, and the Church and the law of suffering. By virtue of this Rationalism and Materialism is torn up by the roots and destroyed, and there remains to Christian wisdom the glory of having to guard and protect the truth. It is moreover a vice common to the enemies of the faith of our time especially that they repudiate and proclaim the necessity of repudiating all respect and obedience for the authority of the Church, and even of any human power, in the idea that it will thus be more easy to make an end of faith. Here we have the origin of Anarchism, than which nothing is more pernicious and pestilent to the order of things whether natural or supernatural. Now this plague, which is equally fatal to society at large and to Christianity, finds its ruin in the dogma of the Immaculate Conception by the obligation which it imposes of recognizing in the Church a power before which not only has the will to bow, but the intelligence to subject itself. It is from a subjection of the reason of this sort that Christian people sing thus the praise of the Mother of God: "Thou art all fair, O Mary, and the stain of original sin is not in thee." ( Mass of Immac. Concep .) And thus once again is justified what the Church attributes to this august Virgin that she has exterminated all heresies in the world.
Source: Ad Diem Illum Laetissimum (Vatican.va)