Colin Wilson said that Existentialism was an attempt to answer the question: "What am I doing here?"
Clearly, there is no answer to this is the mainstream ideology of The West (whatever we want to call it) because it assumes there is neither purpose nor meaning to anything; but neither is there any answer from traditional forms of Western Christianity - which focus, almost exclusively, on salvation as The Problem of life.
And for mainstream Christianity, salvation really is the problem, being terribly difficult and insecure; such that it (ideally) requires constant attention and unremitting effort.
For a traditional Christian; we can never be sure of our salvation - and therefore a serious Christian can never get much beyond working on salvation.
Yet when "What am I doing here?" is the question; salvation is not an answer.
This, especially when salvation is uncertain and insecure, But even at best, salvation merely kicks the can, moves the problem along.
Why is salvation not an answer? Because - as usually taught - salvation says that the job of this mortal life is... to be saved-from mortal life.
In other words, we live in order to be rescued from living! What kind of 'answer' is that?
The obvious problem is that this answer evokes the question of why we live this mortal life, if this mortal life is so fundamentally unsatisfactory that we need to be saved from it?
Why does God bother placing us into this earthly mortal life At All; when the only point of life is to be rescued?
Why not create us straight into the after-life?...
Yet, even then, we will sooner-or-later want to know what is the point of the after-life.
One who was created directly into Heaven might still (assuming he was a free agent) ask the same question of "What am I doing here?"
Answering the existentialist question therefore requires a very deep answer, if that answer is to be satisfactory.
There is the immediate aspect of "What am I doing here?" - which demands an explanation for this mixed and mortal life on earth; and there is the deeper aspect of "What am I doing here?" - which concerns the fundamental reason for being as it applies to me, personally.
I think I have said enough here to demonstrate why secular existentialism could never answer its foundational question; because the question arose from the unsatisfactoriness of this mortal life; yet secular existentialism excluded - by assumption - the after- and eternal-life which might, in principle, make sense of this finite and entropic mortal life.
Secular existentialism excludes a priori any possible answer to "why?" questions.
The answer is that our personal life exists within God's creation (which has a purpose) and that we matter as individuals because God is a loving parent to us - whose primary concern is with individual persons.
Our mortal lives on this finite earth are therefore both part of the whole scheme of things and also individually significant - and the human condition has been set-up on that basis.
So; to understanding 'why?' questions about our own mortal life, we must eventually discern an answer to "why?" God created creation.