Sartre abandoned what most philosophers before him had said and denied that human nature existed. In the book "Existentialism is a Humanism," he said that because no one created humans, no blueprint or ideal exists that defines human nature. He wrote that "Man simply is. [...] Man is nothing else but that which he makes of himself."
Christianity maintains that God created humans, and therefore a divine idea or plan defines human nature. Sartre illustrates the Christian belief with the picture of an artisan making a paper knife according to a pre-existing formula, an "essence." The essence comes before its existence in Christianity. In Sartre's non-Christian existentialism, existence comes first.
Jean Paul Sartre's views on human nature oppose those of other philosophers, including other existentialists such as Soren Kierkegaard. Even Voltaire, an atheist, spoke of human nature as something with an essence that comes before existence, and Kant said that every human -- whether "wild" or "civilized" -- shares the same fundamental qualities. Sartre explicitly denies these views.