Modernists are the worst enemies of the Church: Saint Pius X

 "Henceforth the enemy of the Church is no longer outside the Church; he is now within." 

Encyclical Esupremi apostolatus, October 4, 1903

2. That We make no delay in this matter is rendered necessary especially by the fact that 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.

St. Pius X, Pray For Us