What are the basic tenets of Protestantism?
That you need to read and interpret the Bible for yourself.
That what matters is your individual relationship to God.
That you will face death alone, and have only your faith with you.
What are the basic tenets of Existentialism?
That you must create your own meaning for the world.
That as an individual, you make your own meaning and morality.
That you are alone in the universe, and must face death alone.
I see these "isms" linked somehow. Perhaps Kierkegaard as the first Existentialist makes perfect sense.
Then again, existentialism as we know it beginning in the 19th century and fully developed in the 20th, has deep roots. Might one consider Shakespeare an existentialist? There are passages of Hamlet and MacBeth that seem to resonate with existentialist thought. Doesn't the writer of "Ecclesiastes" sound like an existentialist? Isn't existentialism rooted in part in Romanticism's primacy of the individual imagination?
The history of intellectual thought is linked in many ways we don't commonly consider.