I was raised an Orthodox Jew, which made Santa Claus rather a moot point, but provided me with any number of other mythical narratives presented as actualities. And I was raised a middle-class American, which provided me (and continues to provide me) with other myths presented as fact.

No particular myth was, as far as I can tell, especially fundamental in the sense of "If I hadn't been exposed to that myth, but everything else had remained the same, it would have all been different."

Of course, it's hard to know for sure.

OTOH, had I been raised in an environment in which no myths were presented as facts, that would likely made a significant difference... but I don't have a clear sense of what that difference would be. I've never met anyone who was raised in such an environment.