In their day, Christianity had not yet become a seperate religion; they would be part of a Jewish sect.
Look, Christians can consider themselves whatever they want. But no. They’re not Jews unless they reject the divinity of Christ as Judaism doesn’t allow for that.
I don’t get this current Christian fetish that some Christians have with trying to pretend that they’re Jews–dudes: have some pride in your own religion and don’t try to dilute mine. I’m not an “incomplete Christian” or a “Christian who doesn’t get it”. You guys aren’t Jews with a little bit of added stuff.
Ecker’s dismissal notwithstanding, it’s not all that tricky–Jews can’t worship a triune deity. Christians do. One’s not better or worse, but they’re incompatible.
You can’t be a Marxist and follow Ayn Rand’s teachings. You can’t be a pro-Communist John Bircher and you can’t be a religious Jew and a religious Christian at the same time. You may think you are, but you’re only doing so by undermining the core tenents of one faith or the other.
Christians: For God so loved the world that he gave his only begotten Son that whoever believes in him shall not perish but have eternal life
Jews: Hear, O Israel, The Lord Our God, The Lord Is One (God).
One God-no Sons allowed. Spare me the Shell/Yolk/White=1 egg metaphor. It doesn’t work within Jewish thought. It does within Christian.
Our religions have a lot in common in terms of history, ethics, morality shared values and those are to be celebrated. But the core vision of who God is is incompatible. And that’s ok.
Christians. (Assuming they thought Christ was the son of God.)
Why is this hard?
If you accept that God has a Son, you are no longer in line with the single most important point of Jewish faith. You’re 180[sup]o[/sup] opposite.
If I start a religion where I say “Y’know…I’ll bet Satan was REALLY God’s Son and he tried to stop the false Son of God sent to turn us from Father Satan. And all who believe in Satan shall have the right to trod on the meek and abuse the sick and helpless”, I couldn’t call myself a Christian without people pointing and laughing at me.
If you worship a God who has three faces/facets/persons/beings, you’re not following the Jewish religion any more.
Why do so many Christians not get this?
Clearly, these are two different religions. The whole idea of Christianity is that there is a new covenant to replace the one that God had with the Jews. Why would you continue to follow the old covenant? Makes no sense.
Now, if you want to still pepper your speech with some Yiddish, and have your Bubbe make you some chicken soup when you’re sick, I’m sure no one would have any objections. But if you believe that Jesus is the Son of God, then you’re not a member of the Jewish religion anymore.
I think the disconnect is that there are two answers to this question. Jews see it as a question about Jewishness while Christians see it as a question about Christianity.
For most Jews, the answer is no - you can’t be both.
For most Christians, the answer is yes - all the original Christians, and Christ himself, were Jewish.
When one side tries to tell the other side how they should see it, inevitably, hurt feelings ensue. It seems to me that each group should just define for themselves how they see the situation and not try to tell other groups what to do.
Groups which are attempting to eradicate Judaism are a separate (and despicable) problem even if they use this question as a jumping off point.
And there you go. Well said and succinct.
Who’s to say that Islam isn’t an offshoot of Judaism as well? “The Prophet did not start a new religion.”
If someone wants to define themselves as a Jew who thinks that Mohammed is the most important prophet, who am I to say that they are not jewish and muslim at the same time?
It’s simply an acknowledgement that religion is arbitrary and can be abitrarily redefined at will. If you want to say Person A isn’t a “real” Christian or “real” Jew (or “really” some combination of the two), go ahead. I just don’t see anything measurable to back up such a determination.
Why, thank you…you’re a mensch. 
If, as is so frequently asserted on the SDMB, a Christian is anyone who claims to be a Christian, then a Messianic Jew is anyone who claims to be a Messianic Jew.
If one objects that to anyone’s claims that “real Christians don’t believe or do (something objectionable)”, then the “no true Scotsman” gets thrown in their face.
And yet, Messianic Jews & their defenders are reviled & slandered with impunity on the boards.
SO I’m dropping the term “Messianic Jews” and just referring to them as Messianics.
To the OP~
Yes, one can be fully ethnically & culturally a Jew & still accept Jesus as Messiah & Son of God.
No, one cannot authentically practice Rabbinic Judaism in its various forms & still accept Jesus as Messiah & Son of God.
The question is- what is the real legitimate version of post-Temple “Yahwism”?
Rabbinic Judaism with Noachism added for the Goyim, Christianity and/or Messianism, or for good measure, Islam?
Btw, there is in some Islamic countries, a type of “Messianic Islam” growing, where believers gather in Friday in ‘mosques’, pray five times daily in the Islamic fashion of completely bowing down on prayer mats, give the zakat/alms, read from the Bible & compatable Quranic passages, refer to God the Father as Allah, and even regard Mohammed as a ‘prophet’ who brought Monotheism to the Arabs but sadly got mixed up regarding Jesus’ Divine Sonship, Crucifixion & Resurrection and the still-chosen status of the Jews (which Orthodox & Catholic C’nity of the time was pretty mixed up on, also, IMHO). Of course, many religious Muslims hate it and Christian reactions range from seeing it as heretically wrong to tragically necessary in a culture where full open embrace of C’tian faith could get one killed.
Judaism is a tribal religion. That means that it has a number of beliefs and customs it considers unique to it. A member of the tribe by birth or by choice doesn’t have to follow his tribe’s customs to be a part of the tribe, but if he starts following another tribe’s customs, he is considered a traitor and cast out.
This, I’m pretty sure you all understand.
What you don’t seem to understand that Jews regard all other religions in exactly the same manner. The whole “religion as a personal philosophy” thing is alien to Judaism - we see religion primarily as a means of declaring allegiance. Christians, to us, are a tribe. If your parents are Christians, then you are a Christian, even if you don’t believe in God and never stepped food in a church. The fact that you don’t see things that way doesn’t change *our *perception of the world.
Do you understand now why you can’t be both a Jew and a Christian? Do you understand why we consider Jews for Jesus a bunch of fucking Quislings?
I think Jews for Jesus are a scam to try and trick Jews (those who really aren’t that familiar with their beliefs, I would imagine), into thinking that you can be Jewish (religiously) and believe in Jesus at the same time. It’s kind of a cult.
It is hard, because Judaism, unlike Marxism, Christianity, atheism, or patriotism, is in some respects an inherited feature and in other respects a cultural feature and in other respects a set of beliefs. The word has multiple definitions. You’re addressing the belief-aspect of being a Jew, and I think you’re doing so correctly. But it looks to me as though according to the inherited aspect of being a Jew, many Jews believe you can no more change that than I could change my Scots-Irish ancestry. When I stopped believing in Christianity, I didn’t stop being Scots-Irish.
How many everyday, ordinary Christians actually want to define themselves as “Jewish”?
As for Christ and his immediate followers - historically, Chistianity developed gradually from being a Jewish splinter group into being a new religion, over a couple of hundred years. But there is zero question that they are, now, a distinct religion - this is a perfect example of an “argument of the beard”.
To my mind there isn’t any two equally legitimate sides here.
They can say what they want. It’s a free country, I can say I’m a the President.
The difficulty lies in getting other people to accept that identity as having some meaning for them.
Sure there is. The fact that the markers of an ethnic identity are kinda arbitrary doesn’t mean they don’t exist.
In its most basic form, a “Jew” is someone who is born of a Jewish mom (or any Jewish parent - opinions vary) or who makes the committment of converting to the tribe.
There is a some minor debate over whether one can cease to be a Jew by converting to something else. Some Orthodox believers believe that you don’t cease to be a Jew by so doing, but the vast majority of Jews believe that, just as one can convert to being a Jew, one can convert from being a Jew.
Otherwise, Judaism would be a bit of a roach motel - if conversion only worked one way. 
Christianity stopped being a branch of Judaism when Paul’s interpretations prevailed. The intellectual antecedents of Pauline Christianity are as much or more Greek in origin than Jewish. Christianity is not “basically Judaism at its core”; it is completely different angle at viewing reality, morality and ethics, even as it comes to many of the same conclusions as to what constitutes moral and ethical behavior and even as it shares some of the same texts.
Can you be religiously Christian while denying Christ’s divinity?
I doubt it, and likewise you cannot be religiously Jewish while accepting it. “Jewish” from the Halachic perspective, yes. Ethnically or culturally Jewish? Sure. You grew up in a Jewish home and it has become part of your fiber. And some Jews will always consider you part of the mishpachah no matter what Church or Mosque you attend. But religiously, you aint both at the same time.
Scotts-Irish, unlike Judaism, isn’t also a religion. For “many” read “The Orthodox”. The majority of Jews believe that, if you convert to Christianity, you aren’t a Jew, but rather someone with Jewish ancestry or upbringing.
I’m not clear on population numbers: are the orthodox a minority, worldwide?
It’s not a cult but it is a scam in a way. It’s a Baptist front group designed to convert Jews to Christianity. It’s literature is misleading. By twisting the meaning of Biblical passages and lying by omission it makes it case to people who are ignorant of the real history behind what was written. Many Jewish people stop their religious studies when they are thirteen years old and have an adolescent view of things. They are easily swayed.
I am a totally secular Jew and an atheist and those guys still chap my ass. I don’t mind Jehovah’s Witnesses or other Christians approaching me but when a “Jew for Jesus” tries to engage me I get infuriated. That said, it was a Jew for Jesus who accosted me when I was around twelve years old in Westwood and started me on my lifelong self taught study of religions and Biblical history. I can debate any Christian to a standoff.