That remains your own assertion. Christians believe that at Jesus time. the Pharisee (and rabbinic teaching) believe that hell exists, redemption from hell also exists. Such ideas can also be found in writings such as the book or Enoch. The Sadducees took another stance though.
"Christians believe that..." doesn't actually mean anything when there are a plethora of actual Pharisaic writings and teachings that make it clear that they did not believe in Hell.
The Book of Enoch is not canonical, and there is no evidence that any significant proportion of the Jewish People ever accepted it as of real religious force.
And the teachings of the Sadducees is irrelevant, since they didn't believe in the eternality of the soul at all, and therefore certainly would not have believed in Hell, or any other kind of afterlife experience.
Pharisees also believed in the resurrection, but
Josephus, who himself was a Pharisee, claims that the Pharisees held that only the soul was immortal and the souls of good people would be
reincarnated and “pass into other bodies,” while “the souls of the wicked will suffer eternal punishment.”
There were a number of different beliefs about afterlife amongst the early Rabbis: there was no uniform doctrine about the afterlife-- and, in fact, to this day there is no uniform dogma about afterlife in Judaism, but rather many ideas and beliefs are reflected.
But none of those beliefs, either in the days of the Pharisees and Tannaim, or today, or at any time between, has included eternal damnation or Hell-- Josephus was either mistaken, or in error, or was mistranslated-- I don't know which.
Even amongst those who have believed in Gehinnom, the purgatory-like afterlife experience that some of the early Rabbis believed in, they almost all taught that punishment in Gehinnom was for no more than a year, and not everyone went there. Only those who had not repented for their sins before death went there, and generally the stay was brief. And historically, the majority of Jews have not believed in Gehinnom at all.
You are right. But the assumption that only the Jews know the truth is.....just an assumption. They believe a form of eternal punishment anyway.
First of all, no we don't. Second of all, we may or may not know "the truth," but we know what the Pharisees and Tannaim did and did not teach.