Well, according to the Testimonium Flavianum, some guy named Jesus was. The authenticity of the document is still in question though. Scholars will argue back and forth on the specific document itself, but not the contents. If the document is then authentic, or possibly authentic, why is it so difficult to accept that a man named Jesus Christ did exist? This does not support the claims of the bible and the miracles of Christ. It merely solidifies that he existed. Even an Atheist can agree with that.
The Testimonium Flavianum should definitely be considered proof of Jesus Christ's existence. Written in the first century by a known historian, it refers not only to Jesus existence, but also contains information relating to early Christianity. Taken into consideration with other documents that have mentioned the existence of Christ, it serves as incontrovertible proof that Jesus Christ lived.
Yes, the Testimonium Flavianum is proof of Jesus Christ's existence, because it provides historical documentation of the life and times around Christ. That Christ appears in other historical writings besides the Bible, like the Testimonium Flavianum, is proof that he existed in the flesh in the real place and time. The document also spoke about Pontius Pilate.
If I say "it is day time", you know it to be true because the sun is shining or false because the moon and stars blanket the sky. If I say it is raining, you know it to be true because of the drops of water falling from the clouds to the earth or false because of the lack thereof. These things are proof for or against my claims. If I say Jesus of Nazareth existed, but my "proof" is a document dated long after his death and whose authenticity is questionable, how so is that proof? It merely becomes a reframed version of faith masquerading behind a thin layer of substance in a vain hope that others might believe my claims. It is no different than telling a lie and hoping the people I am talking to believe it without question.
I do not believe the Testimonium Flavianum is proof that Jesus Christ existed. The manuscript is thought to be written between 93 and 94 AD, which is too far past his death. Secondly, we do not have the full text available and do not know of all of it's original content. Thirdly, we are not even sure if it is authentic.
Jesus was an historic figure because he was mentioned in several historical accounts, not just the Testimonium Flavianum. Jesus is mentioned in Hebrew, Arabic and Gnostic texts in addition to the four Gospels and the writings of the apostle Paul. If you are asking is this proof of Jesus as the son of God who was resurrected on the third day, no one can prove that whatsoever because that is a matter of faith.