Existentialism and Humanism

In his text, Sartre says that the key defining point of Existentialism is that the existence of a person comes (chronologically) before his or her essence[?]. In simple terms, this means that, although that person exists, there is nothing to dictate that person's character, goals in life, and so on. Only the person themselves can define their essence. To quote Sartre:

Man first of all exists, encounters himself, surges up in the world - and defines himself afterwards.

Thus, Sartre is rejecting what he calls "deterministic excuses", and claiming that all people must take responsibility for their behaviour.