If someone comes up to you claiming to be “on a mission from God”, how can you tell whether they’re telling the truth? One way might be to ask them to perform some sort of miracle to prove their special mission. But say they claim to have performed a miracle by turning water into wine at a wedding, as Jesus did in John chapter 2. You are at the wedding, and you can taste the wine, but if you are not the sort of person to believe in miracles, you are going to come up with some other explanation. Someone must have switched the containers, or maybe there was some sort of exercise of mass hypnosis.
People who are determined to believe in miracles will see miracles everywhere, and people who are determined not to believe will find some other explanation, regardless of how “incontrovertible” the evidence appears to be. Even if the sceptic cannot come up with any rational explanation, that will not stop him or her from believing that there is a rational explanation — it just has not yet been discovered. The non-believer takes the same approach to explaining the origins of life and the universe: everything has a natural explanation whether or not it has yet been discovered.
So when some people approached Jesus, as reported in John chapter 2, and requested a miracle to “prove” his right to act the way he was acting, much less would have been accomplished by a miracle than you might expect. If the people were determined to disbelieve, a miracle was not going to change their minds, no matter how impressive. Jesus did offer a miracle, but it was one which would only be understood by those who believed.