I know that Jesus was essential. I know that when he was born into this world, somehow everything in the universe changed.
I also know that this was not primarily a matter of sin - it really wasn't: that is a narrow and partial misunderstanding of 'saviour'.
Christ was a transformation of possibility - so great that I can't summarise it.
I can't do much more than perceive an outline in a blurry fashion... Or rather, I perceive it all but blurred and stretching into a distance, such that I know there are more vital details beyond my clear vision, but can't distinguish them.
Christ did so much. And that continues. It is by Christ that we continue to experience meaning, purpose, miracles - that we rest upon love - that we know that creation is good, and that we are part of it and can participate in it.
All this happened because of Jesus and could not otherwise have happened.
The reason (or reasons) why Jesus is essential are very difficult to comprehend and express - and yet that knowledge is so strong and sure... it is only the failure to explain it that threatens the knowledge itself.
You ask why Jesus is absolutely and uniquely essential - I fail to provide a satisfactory answer (I am not happy with the answer, you are not happy with the answer) - My failure is seen as falsifying the assertion...
If Jesus was subtracted from history, so that he never was, and never was going-to-be... If he had never been part of the plan... If life and the universe had been made only by God the creator and nothing else... then life would have been futile.
Life would have gone nowhere, achieved nothing: it would not have been worthwhile.
Life would, indeed, have been A Bad Idea.
It is Christ which/ who made and makes it All worthwhile.
Somehow the scope and scale of the reality of What Jesus Did has been missed, has been metaphysically denied, has been wrongly-explained. We've explained it to ourselves (for 2000 years) in various partial and distorted ways... and then we have been trapped-by our own limited explanations...
We cannot communicate it, not even to ourselves - yet we insist on the primacy of communication, and insist on living-by that which can be communicated... The tail is wagging the dog.
There was creation - ongoing; and there was Jesus Christ - ongoing. The first made everything possible; the second made it worthwhile.
But ask me to say why and how in a few comprehensible words - or say it in many incomprehensible words - and I cannot say anything that does not grossly misrepresent the fact and its meaning.
We cannot even explain the reality of one single person to another single person. Is it surprising we cannot provide a general explanation of Jesus Christ suitable for humanity?
But we can get-to-know a person.