Professional historians really hate to use absolutes.
Because things happened when they couldn't see them, they don't usually say that something absolutely occurred or absolutely didn't -- instead they use levels of "confidence" in the descriptions we have of events and/or people.
When there's one source that recorded an event in the past, their confidence is low. Their confidence is also low when the only records we have came from people who didn't actually witness the event in question -- second hand reports are always less reliable than eyewitness accounts. They also consider the motivations that the writer may have had to "color" his record -- did he/she have a vested interest in telling a story a certain way? Were they a fan/friend/associate of the person they're writing about, or were they an enemy or a critic of the person? Such motivations also affect the confidence in the details of a record.
So, for example, if we talk about Julius Caesar...we have things he wrote himself (gives high confidence in his actual existence, and a good insight into his own thoughts). We have literally thousands of written records by eyewitnesses to the events in his life, some from friends and some from foes -- where they agree, confidence is high in the accuracy of the accounts because the biases even out. Where they disagree, confidence is lower -- unless there are other sources to refer to, we can't be very confident which of the conflicting stories is more accurate. Using all of the various sources, with careful study and cross-referencing, a history of Julius Caesar can be put together in which we can have fairly high confidence that it's accurate.
Now lets look at jesus: we have no writings from jesus himself. We have no eyewitness accounts of any kind (no, none of the gospels are eyewitness accounts). We have no records of jesus from those who would have been his "enemies" during his claimed life (Romans, Jews, etc.) during his lifetime. What we *do* have are a few *at least* second-hand accounts, from non-eyewitnesses, who have a vested interest in making him appear divine, good, to be a son of god, etc.
Using historical analysis, we can have very little, if any, confidence in the biblical records of the jesus character.
Peace.