I'm hesitant to write this stuff, because I really don't want to offend anyone, but I think from the standpoint of many of the Jewish mamas on this thread we sort of feel like we're beating our heads against a wall. Each and every person is welcome to practice spirituality however they wish, but from the viewpoint of normative Judaism (which I think is what the OP was asking about) belief in a messiah having already come places you outside the belief system of normative Judaism. While you (the generic you) might be technically Jewish (if your mom was or you converted) you are not practicing normative Judaism.
Since rabbinic Judaism is normative Judaism (and that includes Reform, Conservative, Orthodox, Reconstructionist, Hasidic and Mitnagdic Judaism!) we get to answer the question "what do Jews believe about who is a Jew."
Asserting to a group of Jews, "no, it's really okay and normatively Jewish for you to have Christian beliefs, Christianity used to be Jewish"--what's your authority for such an assertion? What is your background in Jewish law and culture? What's the logical connection between the historical Jewishness of Christianity and Christianity today?
It's really annoying, if also a bit comic, to be on thread after thread in which non-Jews tell Jewish people what they believe.