Titus Flavius Josephus (37-100) was born Yosef ben Matityahu. This younger contemporary of Paul was a Jewish general during the first war against the Romans. He was captured by the Romans and was able to find patronage from the future Roman Emperor. Flavius is the Roman name he took from that family.
Josephus was a Jewish historian, best known for his works The Jewish War and Antiquities of the Jews. While very important for reconstructing the events of the first century in general, Josephus is relevant to historical Jesus studies because of his extra-biblical mention of John the Baptist, James the brother of Jesus and Jesus himself. Mythicists rejects these as forgeries, especially the Testamonium Flavianum.