Atheists accuse Christians of resorting to mystery as a last-ditch excuse for our irrational faith in Christianity. And I do think some Christians prematurely play the mystery card.
However, it would be a big mistake to think this appeal is limited to Christians. Atheists do the same thing when they find themselves in a bind. That’s how they deflect Christian criticism. For instance:
There might be perfectly good reasons why you can’t imagine a solution to the problem of consciousness. As the philosopher Colin McGinn has emphasized, your very inability to imagine a solution might reflect your cognitive limitations as an evolved creature. The point is that we have no reason to believe that we, as organisms whose brains are evolved and finite, can fathom the answer to every question that we can ask. All other species have cognitive limitations, why not us? So even if matter does give rise to mind, we might not be able to understand how.
Notice that this is viciously circular. The inability to explain an evolutionary theory of consciousness is chalked up to…evolution!