Icy...this "how do you know" challenge is a favourite of theists, as if everyone is subject to fantasy existence.
Plato can be corroborated by multiple existing historical documents, his own writings, Diogenes Laertius' official records, Socrate's writings etc. Now while these records are hardly the pinpoint accuracy that Google.com is for example
the multiplicity and parallels of said documents would lend to the truth of the claim of Plato's existence.
There is however, no official record of a Jesus Christ DURING his own life. Even those who wrote about him after tended to say Jesus revealed himself through "divine revelation".
Personally, I'm pretty sure that a character, most likely the leader of early christianity, existed and was incorporated in the Bible. However, this book hardly constitutes a historical record; most of it has no supporting independent documentation (heck most of it is quite fanciful). Even the most meticulous record keepers of the time had written of no 'saviour' that went around performing miracles. That
would have been noteworthy would it not?
Point is Icy, are you going to challenge everyone who doesn't agree with you on every single person that they quote? Maybe no one existed before us right now Icy...we could have appeared just this second with false histories implanted in our heads...