I would like to say to Celsus, who represents the Jew as accepting somehow John as a Baptist, who baptized Jesus, that the existence of John the Baptist, baptizing for the remission of sins, is related by one who lived no great length of time after John and Jesus. For in the 18th book of his Antiquities of the Jews, Josephus bears witness to John as having been a Baptist, and as promising purification to those who underwent the rite. Now this writer, although not believing in Jesus as the Christ, in seeking after the cause of the fall of Jerusalem and the destruction of the temple, whereas he ought to have said that the conspiracy against Jesus was the cause of these calamities befalling the people, since they put to death Christ, who was a prophet, says nevertheless— being, although against his will, not far from the truth— that these disasters happened to the Jews as a punishment for the death of James the Just, who was a brother of Jesus (called Christ),— the Jews having put him to death, although he was a man most distinguished for his justice. Paul, a genuine disciple of Jesus, says that he regarded this James as a brother of the Lord, not so much on account of their relationship by blood, or of their being brought up together, as because of his virtue and doctrine. If, then, he says that it was on account of James that the desolation of Jerusalem was made to overtake the Jews, how should it not be more in accordance with reason to say that it happened on account (of the death) of Jesus Christ, of whose divinity so many Churches are witnesses, composed of those who have been convened from a flood of sins, and who have joined themselves to the Creator, and who refer all their actions to His good pleasure.
Source: Against Celsus (New Advent)