To untangle whether or not a Christian may also be a Jew, one must be clear on the terminology.
The way I put it is this: a Jew who is a Christian may still be a Jew by birth, but is definitely NOT practising Judaism, and in fact, is violating a tremendous number of Jewish laws by practicing Christianity.
Jewish law cannot be nullified by some weasel words. Heck, even Reform Jews won't go so far as to say "Sure, go ahead and worship Jesus, it's cool."
It is not my place as a Jew to sit in judgement of other Jews. God is the greatest Judge and one whose power I will not seek to usurp. My understanding is that "once Jewish, always Jewish". Even if a person is banished from a Jewish community, he is still a Jew. So I will leave the question of whether a Christian is still a Jew as moot.
However, I can say with certainty that a person who practices Christianity, particularly the Eucharist ritual which is about as UN-kosher as it gets, is definitely NOT practicing Judaism in any way, shape or form.
I am also certain that Christianity is not at all "compatible" with Judaism, and that those who represent that it is somehow possible to practice them both at the same time is the foulest of lies.
You don't have to take my word for it, just read the Torah.
I died laughing.
Frankly, that depends. Is Christianity compatable with modern, Talmudic Judaism? Probably not, given that the latter is a legalistic system (by which I mean that it says one must follow 613 laws and all of the corollaries in the Talmud to be right with God), and the former is a relational system.
However, the real question is whether Christianity is compatible with Judaism as it is presented in the Tanakh, sans the mountain of additional commentary and legalism. I am fully convinced, after long study, that not only is it compatible, but that without Christ, there is no evidence at all for Judaism--and you have to throw out several books of the OT as being false prophets to boot. I know quite a number of Jews who happen to agree, and who, seeing absolutely no conflict between the Tanakh and the New Testament, see no reason to give up their Jewish identity simply because they trust in a Jewish Messiah.
The question has been asked several times, and I think it deserves an answer: What then of atheists who identify themselves as Jews? And why does the Jewish community, which embraces atheists and agnostics even to the point of giving them dual-citizenship in Israel, condemn only Messianic Jews as having left their faith and community? It's a horrible double-standard.
Now, regarding your objection, how exactly is eating unleavened bread and drinking wine (or grape juice) un-kosher?
I also found this statement by you back in post 116 to be completely illogical, if a very common sentiment: The only "One True Faith" is that which leads you to God. How can this be? If you have two faiths that point to the opposite directions (like Christianity and Buddhism, for example), how can they point to the same God? If you have two faiths that are similar on a lot of points, but which are incompatible in their core beliefs, like Christianity and Judaism, how can they both lead you to God?
Frankly, I think we need to elimiate that kind of sloppy thinking in any honest dialogue about religion. The "all roads lead to God" approach sounds concillatory, but it's ultimately idiotic. While it is possible that God could make leeway for those who are honestly mistaken (though I personally don't believe this to be true for a variety of reasons that I don't want to get into right now), if we're really serious about loving Him, we should be prepared to constantly re-examine our beliefs about how He relates to us and how we are to relate to Him. To do less, to stifle the discussion with vague "oh, it doesn't matter" statements, dishonors God by making our own comfort a god before Him.
Having said that, I also believe that we need to have these discussions with all love and consideration for each other. Having said that, I greatly appreciate your mostly gentle tone throughout this thread, which I have been lurking on since yesterday. We can disagree without being disagreeable.