Josephus
A) simply reported what people told him (Christians)
B) His document was modified.
B-1) For one, in the part that begins talking about christianity, the writing style suddenly changes into terms that he never used before (and terms that weren't even contemporary for use toward Christians, such as tribe). Josephus - not a Christian - is somehow expected to have written things like "if indeed one out to call him a man" and "he was the messiah". If Josephus believed that, why was he not a Christian (as Origen specifically stated)?
B-2) Josephus doesn't even feel fit to mention Jesus or Christians in earlier works on the subject.
B-3) The arabic version of testimonium flavianum - which is older - offers a different, less embelished version of the account of Jesus:
"At this time there was a wise man who was called Jesus. And his conduct was good, and he was known to be virtuous. And many people from among the Jews and the other nations became his disciples. Pilate condemned him to be crucified and to die. And those who had become his disciples did not abandon his discipleship. They reported that he had appeared to themafter his crucifixion and that he was alive; accordingly, he was perhaps the Messiah concerning whom the prophets have recounted wonders."
Even the arabic version was likely embellished, but not nearly so much as the Greek.
C) The oldest manuscripts of testimonium have the particular passage inked out or cut out. Josephus was frequently fairly insulting to proported messianic figures. To believe that this was done by *non-Christians*, one would have to believe that they had a particular grudge against Christians in particular, but not against other groups. Hardly likely.
In short, we have a text that is completely not in Josephus's style, contradicting an otherwise concordant arabic version of the text, where the arabic versionis older, and Josephus clearly didn't believe what was claimed that he wrote or he would have been a Christian, some of the words used weren't in use at the time in reference to Christians... you do the math. For much more detail on the subject, read
this
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"Illuminant light,
illuminate me."