"Existential Humanism" is a concept that can be understood in several different ways. Sartre said "Existentialism is a humanism" because it expresses the power of human beings to make freely-willed choices, independent of the influence of religion or society. Kierkegaard suggested that the best use of our capacity for making choices is to freely choose to live a fully human life that incorporates traditional human structures such as marriage. Camus, in his book The Plague, suggests that some of us may choose to be heroic, even knowing that it will bring us neither reward nor salvation, and Simone de Beauvoir, in her book "The Ethics of Ambiguity," argues that embracing our own personal freedom requires us to fight for the freedoms of all humanity.
Man possesses a human nature; that "human nature," which is the conception of human being, is found in every man; which means that each man is a particular example of a universal conception, the conception of Man.
That kind of humanism is absurd, for only the dog or the horse would be in a position to pronounce a general judgement upon man and declare that he is magnificent, which they have never been such fools as to do - at least, not as far as I know.
Existentialism is not atheist in the sense that it would exhaust itself in demonstrations of the non-existence of God.
Existentialism is a philosophical movement that views human existence as having a set of underlying themes and characteristics, such as anxiety, dread, freedom, awareness of death, and consciousness of existing, that are primary.
For existentialism, human beings can be understood only from the inside, in terms of their lived and experienced reality and dilemmas, not from the outside, in terms of a biological, psychological, or other scientific theory of human nature.
Existentialism decisively rejects this argument, asserting instead that as conscious beings we always find ourselves already in a world, a prior context and history that is given to consciousness and in which it is situated, and that we cannot think away that world.