The Testamonium Flavianum is a passage from Josephus’s Antiquities of the Jews that mentions Jesus. It reads:
About this time there lived Jesus, a wise man, if indeed one ought to call him a man. For he was one who performed surprising deeds and was a teacher of such people as accept the truth gladly. He won over many Jews and many of the Greeks. He was the Christ. And when, upon the accusation of the principal men among us, Pilate had condemned him to a cross, those who had first come to love him did not cease. He appeared to them spending a third day restored to life, for the prophets of God had foretold these things and a thousand other marvels about him. And the tribe of the Christians, so called after him, has still to this day not disappeared.
This has been a popular passage for Christians but there is a problem. It sounds too Christian for a non-Christian Jew to write. There are three basic interpretations of this passage: 1) It is authentic as it is, 2) Josephus did write something about Jesus but was later elaborated by a Christian, and 3) It is a complete forgery. Mythicists tend to take the third alternative but most scholars accept the second, that is there is a core part of this that is original to Josephus but includes later Christian additions.