Here are
two posts I have made previously on the topic. They mostly say what I would tell you here.
As for Jews for Jesus ("Messianic Jews"), traditional Judaism holds that their beliefs are essentially part of Christianity, and therefore are heretical and unacceptable to Judaism. Obviously, non-Jews who wish to practice some of the commandments and syncretize them with Christian teachings are welcome to do so-- that is their own business; but such practices are forbidden to Jews. Our tradition teaches that the messiah has yet to come, and must do certain things to be the true messiah; and it also teaches that Christianity may be a perfectly acceptable way for non-Jews to interact with God, but it is not acceptable for Jews, because we are to practice Judaism.
Christianity took the Hebrew term
mashiach, and in turning it into the term "messiah," or Greek
christos also deeply altered the theological and philosophical meanings of messianism and what a messiah was, and what such an individual was supposed to do. In the Jewish tradition, the mashiach was originally very much a political and social leader. He would be a restorer of the Davidic monarchy, and someone who regained Jewish political autonomy in Israel, and returned the people to the fullest possible practice of the commandments. The Christian notion of the christ as spiritual redeemer from original sin is utterly foreign to Judaism, since we have no doctrines of original sin and salvation. The notion of the christ as the literal Son of God (much less a facet of God incarnate) is utterly incompatible with Judaism, which holds that God is entirely transcendant and only spiritually immanent; and does not reproduce in such a fashion, or ever incarnate in any way, or appear as an avatar of human or animal form.