Main belief: existentialism can be atheistic, theological or agnostic. Then there is traditional philosophical views where it was believed that "at first man was nothing, but only afterward will the person be something and the person will do things to make up who they are and meaning in there world. Along with rationality was seen as a bad faith because it was a mechanism by humans that counter their existential anixety.
Existentialism appeared in the early buddhist and christian writings and in its current form was inspired by the 19th century philosophers,writers and Czech. Then existentialism became of age in the mid 20 century largely through the scholarly and fictional works of the french existentialists. Along the Sartre possibly being the most well known person to have accepted being called an existentialist.