As a grade 7 atheist, even the appearance of a god would not convince me. Quite simply gods cannot exist. My argument is that if a god appeared and gave me (or us) the scientific explanation of how he did apparently miraculous things, then he would no longer be a god. I liken this to seeing a bird fly when I cannot fly -> once I know how it is done, it is not that remarkable.
This stems from Clark's third law, "Any sufficiently advanced technology is indistinguishable from magic."