Our exploration of 20th Century Philosophy today takes us into the heart of existentialism, a major philosophical movement that has shaped our contemporary understanding of human freedom, subjectivity, and the meaning of life.

To begin, let's introduce existentialism. Drawing on its Latin roots, 'existentialism', broadly defined, refers to the philosophy focused on existence, specifically, human existence. Unlike many other philosophical schools, existentialism does not emphasize a systematic theory, but rather confronts the illogical, paradoxical, and subjective aspects of human life.

The origins of existentialism can be traced back to the 19th century, more specifically to Danish philosopher Soren Kierkegaard and German philosopher Friedrich Nietzsche, although the term "existentialism" did not come into official use until the 20th century with Jean-Paul Sartre. Both Kierkegaard and Nietzsche were concerned with subjective human experience and critiqued the abstract rationalism of their contemporaries.

Now let's delve into the main themes of existentialism. Perhaps the quintessential existentialist axiom is Sartre’s phrase "existence precedes essence." By this, existentialists mean that humans first exist and only then, through their actions and choices, define who they are. This assertion emphasizes the fundamental freedom of the individual but also the accompanying responsibility, revealing the inherent tension of existentialist thought.

Another key existentialist theme revolves around absurdity and despair. Here, life, devoid of any predetermined meaning or purpose, is seen as absurd. The realization of this absurdity can lead to despair—a profound existential crisis— but also, paradoxically, to the potential for authentic living.

A third theme in existentialist thought pertains to authenticity and bad faith. Authenticity involves acknowledging the absurdity, freedom, and isolation of one's existence and embracing the resulting responsibility. On the contrary, bad faith involves self-deception, denial of freedom, and evasion of responsibility.

Importantly, existentialism also provides a critique of societal conventions and institutions that limit human freedom and authenticity. This critique led to diverse applications of existential thought, influencing not just philosophy but also literature, arts, and psychotherapy.

It is important to remember that existentialism is not about hopelessness or nihilism. Despite its sober view of existence, at its heart, it affirms the possibility of creating meaning and value in an indifferent universe. Though hinging on high intellectual discourse, existentialism at its core is remarkably accessible—it asks, and attempts to answer, timeless questions about the human condition, questions we all grapple with in our quiet moments. Thus, existentialism remains a profoundly influential force in our modern philosophical landscape, compelling each of us to confront our existence and search for authenticity in our individual paths.