Many people of all walks of life, including the most famous and brilliant scientists believe that the possibility of a Creator/God as well as the scientific origins of life are beyond our human understanding and knowledge, but they are not Theists as you referred to. They are just intelligent enough to concede of the possibility of realities beyond are limited human knowledge capacity.
I think you're playing fast and loose with the word "possible." Normally, most people take "possible" to means something more than just "not entirely ruled out."
When we're talking about the question of whether something we dream up exists "beyond human knowledge," the answer is always
"we can't completely exclude the possibility, but we have no reason to believe that it exists... or even that it necessarily could exist."
(Unless the "something" is internally contradictory; in that case, we
can say "no, that thing doesn't exist... not even off somewhere beyond our knowledge.")
And that answer is the same for anything we can dream up. You have no reason to give greater consideration to a god we know nothing about than to any other thing that you can't
absolutely rule out as impossible.
All the stuff that could inform any suggestion of a god - religious scriptures, miracle claims, inference of god(s) from "nature", etc. - isn't available to inform your decision to elevate this god you know nothing about above all the other stuff you know nothing about, since by arguing that God is "beyond human knowledge," you're implying that everything within the scope of human knowledge
edit: that purports to be knowledge of a god - including those religious sources - is entirely fabricated and is therefore absolutely useless to inform us about the god you're suggesting.
... so your "god beyond human knowledge" has nothing more going for it as something worth our consideration than anything else we can claim is "beyond human knowledge" and can't be excluded as false... including Russell's Teapot, Sagan's invisible dragon in the garage, and a planet on the far side of the galaxy that's ruled by a civilization of intelligent ice cream treats.
So what reason do you have to suggest that your creator-god deserves more of our attention and regard than the Klondike Bar Planet? None... because you've declared that nothing we could use to justify giving your god greater regard is available to us.