As a lot of people have said, there are many people who were mostly only written about after their death in the ancient world. However, there were people who wrote about Jesus around the time of his existence and his execution that were not Christian like Roman historian & senator Tacitus. One of the passage of his work Annals talks about the execution of Jesus:
"called Christians by the populace. Christus, 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 Pilatus."
Scholars generally say that it is authentic since many of the events mentioned in the book also match up with records of other events at the time. Historian William L. Portier even stated that there was consistency between the references by Tactitus, Josephus (a Jewish historian who wrote about Jesus), and letter from Pliny the Younger to the Emperor Trajan (where he wrote about "Christus", which makes since many historians like Charles Guignebert don't doubt that Jesus lived in or or near Gallilee in the first century). So historians generally agree with evidence like this that Jesus probably existed and wasn't just a myth: it is just simply a possibly that a popular Jewish teacher who was executed as a heretic was mythologized (which was pretty common in the era and even in times near common day, like how George Washington has been worshipped as god by Hawaiian Shintoists for years even though most of us see him as just a man & how the inventor of the language Esperanto L. L. Zamenhof was made a spiritual deity in the Oomoto religion).