Jean-Paul Sartre was a French existentialist philosopher, playwright, novelist, and political activist. He was one of the most important figures in 20th century philosophy and existential thought. His principal philosophical work was Being and Nothingness, which helped make him a household name. He won the Nobel Prize in literature in 1964. During his lifetime, Sartre had a larger following than any philosopher throughout history. 
A passage from Existentialism and Human Emotions:
"If man, as the existentialist conceives him, is indefinable, it is because at first he is nothing. Only afterward will he be something, and he himself will have made what he will be. Thus there is no human nature, since there is no God to conceive it. Not only is man what he conceives himself to be, but he is also only what he wills himself to be after this thrust toward existence." 
"I tell you in truth: all men are Prophets or else God does not exist." - from his play, The Devil and the Good Lord (1951, Act 1). While not every statement made in his plays can be said to represent Sartre's personal opinions, he used his plays as a vehicles to espouse his beliefs, so many of his beliefs were included.