Deep philosophy

What is existentialism? my standard reply to this is by saying existentialism is the philosophy of existence that emphasizes the existence of the individual person as a free and responsible agent determining their own development through acts of the will. I'm talking about existentialism because apart of Marxism there is being no philosophy that has such a radical impact on the western culture as this philosophy has had in the last 50 years or so. The philosophy the is called existentialism has it roots in the 19-century they are two kinds of it, on the one hand, there is a pessimistic existentialism or atheistic existentialism which is distinguished from an existential movement that aimed to set a synthesis with historic Christianity and the most important figure of the in existentialism in the 19 century is the Danish philosopher Søren Kierkegaard and many regards as the father of modern existential thought at least his impact on theology. the pessimistic existentialism has