So atheists, if Jesus didn't exist, why did Rome outlaw Christianity?

Yes, it was a very frightening religion to Romans, because it told slaves they had worth (until later authorities figured out that since that worth was all in the afterlife, it was okay to promulgate the creed after all). They did outlaw Judaism in 70AD after the destruction of the temple. But even after being outlawed and enslaved and dispersed from their homeland, they refused to give up Judaism. Guess that makes it true. Oh, wait, the Romans also outlawed the Cult of Cybele (partly because the priests would voluntarily castrate themselves; nobody would do THAT for a goddess that wasn't real, right?). By the by, before Nero, it was mostly umed that Christianity was just another Messianic Judaic cult, by the Romans.