Well, I don’t want to oversell Josephus. Everybody thinks that the person they work on is the most important—so let me just say that he’s by far, the single most important author that anyone should come to grips with.
Why? Because he, first of all, lived at the same time that the New Testament texts were being written. He was born in 37, died around 100, (C.E.) which makes him a contemporary of virtually every New Testament text. He lived in Jerusalem just from a few years after Jesus’ death to his adult life, and at age 34 he moved to Rome and arrived there just a few years after Paul was probably executed there under Nero.
Then he proceeds to write 30 volumes of Judean culture and history and he tells us virtually everything we think we know about the environment of the New Testament. So—who were the Pharisees, Sadducees, Essenes, Pontius Pilate, Caiaphas the high priest, Herod, Herod’s sons and grandsons, all of these rulers and figures and groups, the temple, the priests—in the background of the New Testament. Most of that information that you see in handbooks comes from the work of Josephus; and in western history he’s had, by far, the greatest impact of any classical author, ancient author, outside of the Bible itself.