Flavius Josephus (37-7? AD), was a Jewish priest in command in Galilee at the time of the Jewish Revolt. After being taken captive, he retired to Rome and wrote a history of the war in Aramaic, of which we have the Greek translation: The Jewish War (75-79). His next work, Antiquities, is a history of the Jews.
While Berlioz claims that Josephus never mentions Jesus, in fact the Antiquities includes a reference to Christ. Because the pro-Christian tenor of this passage sounded odd in the writings of a Jewish priest, proponents of the Mythological School tried to show that the passage was a later insertion by Christian writers. But by the time Bulgakov was writing, the authenticity of the passage had been proven, showing that Christ was indeed a historical figure.
Antiquities, Book 18, Ch. 3 :
Another text from a 10th Century Christian Arab chronicler preserves a Syrian copy of the Greek original. It includes the assertion that "it is possible that he was the messiah about whom the prophets foretold miracles." (I. D. Amusin, "Ob odnoi zabytoi publikatsii Tartuskogo professora Aleksandra Vasil'eva," Uchenye zapiski Tartuskogo GU, 1976, vyp. 365)
The full texts of Josephus are available on the Josephus Page.
See also the Catholic Encyclopedia on Josephus.