Modernism

The Three Great Skeptics came in the mid-to-late 19th century and were against the theories of the enlightenment. As such, they rejected many of the things their worlds were built on - democracy, nationalism, capitalism, and so on. The common link between these three men is that they don't believe, as Hegel does, in a central narrative to the world. Everything is subjective. It is in a way the philosophical solidification of the romantic ideas Rousseau began espousing a century earlier.

Marx has already been talked about, but his ideology was essentially that the lower classes were constantly being lied to and manipulated by the upper classes. Between these two classes, there was constant warfare. And, he heavily implied, it was the geist of the lower classes that would eventually trump the upper class.

Nietzche believed that mainstream culture was corrupt and doomed. Morals are subjective. For example, the statements that “it is wrong to steal” or that “charity is good” are not facts. These things have specific history as ideas and aren't universally true except for specific cultures, and within cultures these ideas can change, as in the example of homosexuality, which was accepted in the Greek and Roman world, considered an evil once Christianity spread, and has once again become an accepted facet of the world, at least in most western countries anyway.

Freud was against the enlightenment because he thinks that we don't know what we want, that we're perverse and that behaviour and morality are all determined by subconscious drives. Sexuality & hunger are the main two. He explains the oedipus complex, the five stages in growth (all sexual in nature) and the Id, Ego and Superego. His essential claim is that we act to fulfil some sort of drive no one is aware of. It is the science of irrationality and anethema to enlightenment ideas. It's for good reason that Freud is considered a dead end in the scientific community, as his 'research' is full of holes. But the cultural impact of his ideas were great. 

Modernism is the theory of relativism. There is no centre, things are only defined by their relationship to everything else in the world. No one is equal, there is no model of perfection to strive for, however we can respect this diversity of such a worldview.