The text in question is a historical text called "Chronicles", which the Greek historian Phlegon wrote around 140 A.D., but, over time, has become completely lost, as have most of his writings. Origen (185-254 A.D.) was a Christian scholar, who, in his written response to the pagan Celsus in 248 A.D., made reference to"Chronicles". Origen says, "Now Phlegon, in the thirteenth or fourteenth book, I think, of his Chronicles, not only ascribed to Jesus a knowledge of future events (although falling into confusion about some things which refer to Peter, as if they referred to Jesus), but also testified that the result corresponded to His predictions." (Origen, XIV) He later quotes a passage from Chronicles, saying, “Jesus, while alive was of no assistance to himself, but that he arose after death, and exhibited the marks of his punishment, and showed how his hands had been pierced by nails.” (Origen, LIX).
Is there any reason to believe that Origen was fabricating these bits, putting words into Phlegon's texts that weren't really there? It's unlikely. Though "Chronicles" is lost now, it wasn't at the time, having been only about a century old. Had Origen been lying about what Phlegon wrote, it would have quite easy for his audience to have found out. Also, Origen was a very respected teacher and philosopher, and has not shown any signs of such dishonesty in his writings. There are also things in those two references which would have been very odd for Origen to have made up, noting that Phlegon had confused Peter and Jesus at times, and saying that Jesus was "of no assistance to himself" while alive.