Existentialism is a philosophy that can be atheistic and theistic. The philosophy views human existence as an independent and as-of-yet meaningless form. The meaning of life is imbued as the individual progresses through life It asserts that morality and truth are subjective . The philosophy states that people create meaning as their lives progress. The name derives form one of its central concepts, the notion that a human being's "existence precedes essence."
The earliest existentialist was Søren Kierkegaard. He was heavily influential in Christian existentialism as he was in ministry for a period before leaving to become a theologian/writer. To Kierkegaard, God was essential to have faith in order to not fall into the absurd angst that is present in the world. Other writers were Jaspers and Heidegger. The movement did not, however, develop its name or popularity until the 20th century when it was popularized by Jean Paul Sartre and Simone de Beauvoir. 
Especially Sartre says, "Existence precedes essence." He asserts that man appeared not from essence (God or an idea) but rather from nothing and then defined himself and gave essence to himself.
The writings of existentialists such as Sartre reflect an epistemology that is not only based upon the criteria of rationalism or empiricism. Theirs is an appeal to the intuitive, as reflected by Sartre’s references to the “nausea” stirred in him by the contradictions and hypocrisy in human behavior. The same appeal to the intuitive can be found in Soren Kierkegaard’s notion of dread and in Albert Camus’ reflections on universal guilt.