Yes, non-Christian religions are false. Christian confessions outside of Orthodoxy are gravely mistaken. But effective missionizing requires calling non-Christians to recognize the prefigurements of Christ in their own spiritual reflection as much as it requires denouncing heresies. Look at St Paul's speech on Mars Hill, where he could have simply gone on about how the Greek deities are demons, but instead said that so far the Greeks were only getting one side of the story, but now he'll proclaim the glory of Christ that was hidden to them.
I was not aware that we were missionizing here in this thread. I thought this was internal family shop talk. I agree that it would be wrong to walk up to a Jew and say "You follow a false religion". Perhaps we're coming from different contexts.
My greater concern is those who seem to have difficulty saying any religion is false, despite the fact that both religions are aware of, and actively reject, the only God there actually is.
And I am not so sure it's the same thing, in any case. Both modern Judaism and Islam are aware of Christianity's existence, and dispute it. This is because they both came into play after Christianity. The Greeks of St Paul's day did not know of Christianity. Obviously that's not an excuse for us not to missionize, but it does change things a bit from the example.
I am pretty certain that Judaism pre-dates Christianity..Just an FYI 
And the Judaism beleived during the time of Jesus is the same religion as it is now. It's just that many of the factions and sects were killed off and we were left only with one or two forms. So no, there are no Sadducees or Essines left, But there are Pharisees left of the type that Jesus made common cause with and others that his strongly opposed. It's complicated just like Christianity has been complicated by the rise and fall of various factions, sects and heresies.
There is a pretty good book put out by the Karaites ( a Jewish Sect) called :
"The Hebrew Yeshua vs. the Greek Jesus" by Nehemia Gordon
Oh believe me, I know. I was heavily into Judaizing forms of Christianity in my formative years. And I've heard more conjecturous teachings about "our Rabbi Jesus" in my life than I care to think about.
Modern Rabbinical Judaism is completely different from Second Temple Judaism. I posit it is, for all intents and purposes, a different religion, in the same way that Mormonism and Orthodoxy are different religions.
Yes, the modern Jewish religion can be traced back to Pharisees and all, I know that. It's not entirely reconstructionist, but where is the Temple? Judaism has been centered on the temple and the sacrificial system since Moses—nay, Cain and Abel. If the Jews claim to be the same religion as the people in the Old Testament, where is the worship of the Old Testament? How are they atoning for their sins if God told them to do it via blood sacrifice? Where is the scapegoat?
I can tell you where the Jewish sacrifice is today: it's sitting on the altar of my parish church, and yours.
The Judaism the Apostles knew was destroyed with the Temple, and those Jews who rejected their Messiah cobbled together a new religion altogether, which reinterprets scripture radically different from those they claim to be spiritually descended from. I do empathize for them; I can't imagine seeing what they considered the dwelling of God destroyed. But the fact is that God had walked among them and they killed him; he came to live within the hearts of his people, and they ignored him.
I'm not saying Jews are bad people, or that they do ugly or evil things. But I am saying that Orthodoxy is Supercessionist, and that the one and only Israel is the Orthodox Church, and the only true "Jews" are Orthodox Christians. I'm not being trimphalistic, I'm merely stating facts. Judaism was entirely superseded by the Church, and there is no truth in Judaism beside the fully-revealed truth in the Church, who is Christ.
