To admit ignorance concerning why there is anything at all is not to concede that we can't rationally rule certain explanations out. It seems to me we certainly can rule out the Judeo-Christian God, as traditionally defined. Whoever created the universe, it certainly wasn't him.
Mark Vernon defends his agnosticism by saying, in effect, "But sophisticated theists don't say God is all-powerful and all-good. In fact they sensibly don't say anything about him at all. So you haven't shown their "mystery God" doesn't exist, have you?" Here's Vernon:
My point is that all images of God must be done away with. So, no: you can’t agree on what a true God is. Again, the great theologians say this: God is always wholly other. You might approximate. But then you have to do away with your approximations too. God is beyond human comprehension else not God.
But now here's my question: what is the difference between the atheist who admits there is indeed a fascinating mystery about why there is anything at all, a mystery to which they do not have the answer, and Vernon's theist who says there's a mystery about why there is anything at all, and calls this mystery "God"?
Surely the difference is entirely trivial and semantic?
By the way, Mark seems to think that the mystery of existence is one that just doesn't much grip atheists. Of course it does. I just spent some time reviewing Bede Rundle's Why There Is Something Rather Than Nothing for Philosophical Review. Rundle is an atheist, yet he's clearly gripped by the question.