Why is Modernism such a grave heresy, why has its influence been so profound? There are objective motives for that: it is the "synthesis of all heresies". Yet, its imprint in the history of the Church in the 20th century (and also in the present century...) is mostly related to the Modernists themselves and to their shameless use of the traditional ecclesiastical structures and offices to further their views and widen the number of their followers. As we prepare for the Centennial Year of the great Encyclical, let us recall the warnings of Pope Saint Pius:
... the partisans of error are to be sought not only among the Church's open enemies; they lie hid, a thing to be deeply deplored and feared, in her very bosom and heart, and are the more mischievous, the less conspicuously they appear.
We allude, Venerable Brethren, to many who belong to the Catholic laity, nay, and this is far more lamentable, to the ranks of the priesthood itself, who, feigning a love for the Church, lacking the firm protection of philosophy and theology, nay more, thoroughly imbued with the poisonous doctrines taught by the enemies of the Church, and lost to all sense of modesty, vaunt themselves as reformers of the Church; and, forming more boldly into line of attack, assail all that is most sacred in the work of Christ, not sparing even the person of the Divine Redeemer, whom, with sacrilegious daring, they reduce to a simple, mere man.
Though they express astonishment themselves, no one can justly be surprised that We number such men among the enemies of the Church, if, leaving out of consideration the internal disposition of soul, of which God alone is the judge, he is acquainted with their tenets, their manner of speech, their conduct. Nor indeed will he err in accounting them the most pernicious of all the adversaries of the Church. For as We have said, they put their designs for her ruin into operation not from without but from within; hence, the danger is present almost in the very veins and heart of the Church, whose injury is the more certain, the more intimate is their knowledge of her. Moreover they lay the axe not to the branches and shoots, but to the very root, that is, to the faith and its deepest fires.
...having struck at this root of immortality, they proceed to disseminate poison through the whole tree, so that there is no part of Catholic truth from which they hold their hand, none that they do not strive to corrupt. Further, none is more skilful, none more astute than they, in the employment of a thousand noxious arts; for they double the parts of rationalist and Catholic, and this so craftily that they easily lead the unwary into error; and since audacity is their chief characteristic, there is no conclusion of any kind from which they shrink or which they do not thrust forward with pertinacity and assurance. ...