The main danger of Modernism is that it affects the very reason of humankind. The modernist mindset leads only to the death of reason, which causes men to quickly descend to the level of disordered beasts (the result of the damnation caused by original sin). When the very foundation of reason is attacked as it is happening in current times, everything is turn upside down and all coherent thought is no more.
The modernist mindset also attacks the very foundation of a "true" and outside - reality, independent from individual thought. It is this objective reality that leads to the knowledge of God, common sense, and Christian morals. By making alternative "realities" and individual "truths" based upon subjective and personal experiences, Modernism inverts the right order of things as imposed by God, and cause irrationality, insanity, and utter decadence.