The platform for existentialism is logic and reason.
However, logic and reason are dependent on God. Let me explain:
If there is no God, then human intelligence is simply the product of a universal accident.
If there is no God, then logic and reason are domains employed with collective human intellect
(collective universal accidents), and is also a universal accident.
Therefore, within a context where there is no God, there is a great probability of error in
any logical assertion as both the domain and the one who makes the assertion are by nature accidental.
If both your user and tool are accidents, the success rate of your product is unlikely to be good.
The best scenario for logic and reason are when a God exists:
A God who designed the human brain and intellect so that humans may have a sense of understanding
of the world (albeit non omniscient).
A God who designed the framework for logic and reason, and himself also is definable by the basic
principles such framework (Infinite, Omnipresent, Immovable, All Good).
In consequent, pure existentialism should be void of reasoning or logic, and should not be philosophical
(as it is philosophically largely open to error, and should not be a clear option). However, by definition, existentialism cannot be so.
What is existentialism without philosophy? Self-centered living.