Well, there are scientists who know an awful lot about the chemistry of wine; certainly much more so than people did 2000 years ago. If it were possible to instantly convert water into wine, we'd know about it, or at least have some idea about how it was conceivably performed.

For the fantastical accomplishments of people of thousands of years ago to be believed, there needs to be more concrete evidence than stories passed down for generations. In this situation, it's unlikely that such evidence survived, so the best we could do is replicate what happened in the past and determine whether or not such an accomplishment could have happened given the technology of the past.

I believe that people 5000 years ago could build awe-inspiring structures from stone because there's plenty of evidence of such capabilities (i.e., the great pyramids). I don't believe that someone turned water into wine instantly, because, afaik that is not possible, thus, at best, the event in question was some sleight-of-hand magic trick.