I think people would be more skeptical, for three main reasons: 1) We have a much better understanding of life, the universe and everything. 2) We also live in a less religious world, in that there are over a billion non-religious people about. 3) I think people would need a lot of convincing of why they needed to be "saved" and what from. 4) Religion has less control over lives now, less public burning! However, if He were to perform genuine miracles and spread His message well then I'm sure people would accept Him, but as ever opinion would be very divided. I don't think Christianity would become very large, as we are living in a world where most religions are decreasing, just some are going quickly, some slowly. Nethertheless, a 21st century Jesus would still have a noticeable effect