I'm no theologin, but in a word, at this time, since Jews believe that he is NOT the messiah, then no, they would no longer be Jews. Again, could a Christian be a Christian if he or she abandons a main tenant of faith?
I realize that this is a tough thing to get used to. With religion, you know you're right. You have to be, it's something that's taken on faith.
Mark
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.