Early Christians wrote about him, Jewish historians wrote about him and secular historians wrote about him.
People wrote about Captain Kirk and Indiana Jones... did that make them real?
Actually, there are no writings about Jesus before the Third Century, and most of the attributions like Tacitus and Josephus are suspected of being later writers inserting text. (As all texts were hand copied before the invention of printing presses.)
For instance, Tacitus says Nero blamed the Christians for the Great Fire in Rome. But this is not mentioned in Suetonius and Dio Cassius. Kind of an important detail to miss, don't you think?
Further, when Pliny the Younger wrote Emperor Trajan for instructions on what to do with Christians when he caught them. (Christians not necessarily meaning followers of Jesus) Trajan recommends being lenient with them. Really? The guys who started the Great fire? This is like someone in 2040 being lenient with ISIS members.