Do we get all our information about Jesus' existence just from the Bible?

No! Both Jewish and Roman writers, including Josephus, Tacitus, Suetonius, and Pliny refer to Jesus as well. None of these men were Jesus' followers, and they mention Him only in passing, but they had no doubt about His existence or impact. Some people have relegated Jesus to the level of Peter Pan, Santa Claus, and the Tooth Fairy.