Is there a difference between Modernism and modernism? How do we know which aspects of modernism are the ones that have been declared a heresy and which are not?
From the encyclical of Pope St Pius X on the doctrines of the modernists: (in keeping with the rule of only posting three paragraphs - you’ll have to read the rest from the link)
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.
And 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. To this must be added the fact, which indeed is well calculated to deceive souls, that they lead a life of the greatest activity, of assiduous and ardent application to every branch of learning, and that they possess, as a rule, a reputation for the strictest morality. Finally, and this almost destroys all hope of cure, their very doctrines have given such a bent to their minds, that they disdain all authority and brook no restraint; and relying upon a false conscience, they attempt to ascribe to a love of truth that which is in reality the result of pride and obstinacy.