The ultimate origin of existence is beyond human scope, so far as we can tell, yes. We can continue to push the existence point further and further back, but there is always an origin point that is likely to remain outside our scope of empirical knowledge.
Ergo metaphysical answers, including the existence of a supreme being always have some validity. If you water it down enough, the existence of a "creator" cannot, and likely never will be, disproved.
However that the aforementioned "creator/supreme being" is as defined by Christian tradition is very, very much up for rational debate.