Not a historian, but I have a follow-up question for clarity. What evidence are you referring to?

I'll be happy to jump in here and say a few things. I apoligize if this is much more info then you were seeking, but I wanted to supply you with the best, well-rounded answer I could give.

First, there is no hard evidence that Jesus of Nazareth existed. We have no documents, monumental inscriptions, annals, or any other type of credible ancient writing from around the year 30 CE that anyone named Jesus of Nazareth existed.

That said, I don't know of any credible historian who specializes in this time period who disputes that the figure of Jesus had to have existed. Whether the historians are religious, like E. P. Sanders or John Dominic Crossan,or agnostic or atheist like Bart Eherman (who actually wrote a book on this topic called Did Jesus Exist: The Historical Argument for Jesus of Nazareth )or Robert M. Price (who also wrote an excellent book on this topic called The Christ-Myth Theory And Its Problems ) all credible historians admit that the likelihood of Jesus not existing is almost impossible.

Now people sometimes like to play the card of doubt, saying that Jesus did not exist (as your question entails) but this mostly comes down to misinformation, in my opinion. I assume many people who have this opinion see some pieces of evidence, such as no early copies of the gospels existing within nearly a century of Jesus’ death. The oldest piece of a gospel is known as P52, which is a small part of the gospel of John which is about the size of a credit card. Experts in ancient handwriting (paleographers) date this text to the first half of the second century, plus or minus 25 years — most estimate it around 125 CE. This is a bit troubling because this tiny sliver of information was still a copy of a copy and was written 30-40 years after John was written. We don’t actually start seeing larger sections of text for most gospels until much later, such as P45, which date to around 220 CE — again, nearly two hundred years after Jesus’ death. Situations like these can easily enable uneducated people to make the assumption that because these texts are so far removed, so incomplete, and also contain an uncountable amount of errors, they are probably just false all together.

Another issue some people have is that the first and only (Non-Christian) person to write about Jesus and his followers was the Jewish historian Josephus, who would only briefly mention Jesus in his twenty plus volume history of the Jewish people called The Antiquities of the Jews. Here Josephus discussed the execution of a man named James, whom Josephus described as “the brother of Jesus who is called the messiah” (Ant. 20.9.1) (I’d also like to note that Josephus never believed in Jesus being the Messiah, he was merely stating what others had said). He does also discuss Jesus again, but I won’t bore you with ancient quotes that would take me far too long put into context. The important thing to realize is that this first mention of Jesus by a non-follower takes place after the year 90CE, so again 60 years after Jesus’ death. If you are wondering why this is an issue for people, imagine people today writing about the USSR launching sputnik? You’d imagine that some of the information since that time would have had to have been misconstrued.

Lastly, I will say that any doubters cite that Pegans, both Roman, Greek, etc. never wrote about Christians until the second century. For doubters, they site this as saying that if Christianity was as significant religion which was founded by a man named Jesus, shouldn’t the Romans have written about it? The historical consensus is no. Not at all.

Contrary to popular belief, Christianity wasn’t a quickly expanding religion until the second century (and even then it did not grow very fast). The first mention of Jesus by a Roman was in 112 CE where (in a letter) the governor of the Roman province of Bothnia-Pontos (where Turkey is today) mentions a group of people who would meet in the mornings before light, who worshiped "Christ as a god" and used to share meals together. In this letter they don't even mention Jesus' name since they are focusing on the actions of his follows. It's not until years later where the Roman historian Suetonius made Life of Claudius ( University of Chicago.) http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Suetonius/12Caesars/Claudius*.html) what many believe was the first indirect mention of Jesus' name in writing (this is of course apart from the Christian writers of the New Testament who had started writing about him for more than half a century prior).

I hope this helped to provide at least some information as to why some people doubt the existence of Jesus. For historians and even agnostic aspiring scholars like myself (I am an Early Christianity graduate student) the idea that Jesus didn’t exist seems impossible. While many historians will debate the true nature of Jesus (what he said/did) and the accuracy of the documents that describe his actions that still exist today, there is too much textual evidence to support the idea that a Jewish rabbi named Jesus caused an off-shoot branch of Judaism that sprung into Christianity.

If you have any other follow-up questions please let me know!

Edits: grammar and clarity.