- With respect to written historical references to Christ, here are some examples:
The Jewish historian, Flavius Josephus wrote in his Antiquities of the Jews, written around 93–94 AD:
Now there was about this time, Jesus, a wise man, if it be lawful to call him a man, for he was a doer of wonderful works - a teacher of such men as receive the truth with pleasure. He drew over to him both many of the Jews and Many of the Gentiles.*
He was (the) Christ; and when Pilate, at the suggestion of the principal men amongst us, had condemned him to the corsscross, those who loved him at the first did not forsake him, for he appeared to themn alive again the third day, as the divine prophets had foretold these and ten thousand other wonderful things concerning hiim; and the tribe of Christians, so named from him, are not extinct at this day" (Antiquities, XVIII, III)* (source)
Tacitus, a Roman historian mentions Jesus in a passage about Nero in his final work Annals written in 116 AD:
Consequently, to get rid of the report, Nero fastened the guilt and inflicted the most exquisite tortures on a class hated for their abominations, called Christians by the populace. Christus [Christ], from whom the name had its origin, suffered the extreme penalty during the reign of Tiberius at the hands of one of our procurators, Pontius Pilate, and a most mischievous superstition [Christ's resurrection] thus checked for the moment, again broke out not only in Judea, the first source of the evil, but even in Rome, where all things hideous and shameful from every part of the world find their center and become popular (link)
Will Durant, a agnostic historian (from what I can find), wrote on historical content:
...the argument took the existence of Christ for granted. The denial of that existence seems never to have occurred even to the bitterest gentile or Jewish opponents of nascent Christianity
- A couple of examples of indirect evidence from historical writing:
- comments in a letter by Roman Pliny the Younger written around 112 AD about the willingness of many Christians to be killed rather than change their beliefs; this would probably be difficult to produce on any scale for a fictional figure
- various quotations of eyewitness accounts of Jesus in Bible sections writings; the tone, context and use of the quotations presume that Jesus existed
- Scientific evidences would be a little more difficult for any historical figure. For example using DNA or fingerprints to prove that someone existed in history is heading toward logically impossible. It seems you would need sample of biometric information from a person for comparison; but then you would, by definition, already know they existed.
Chapter 9 of Gary Habermas' book "The Historical Jesus" addresses this question extensively.