First define what Judaism is, its essence -- make sure it is a definition that the everyone is going to agree with. IOW you can't just make up a definition that is idiosyncratic to you, you don't get to do that, you have to actually use a working understanding of the English language and knowledge of the religion.
Second, show how your own personal Judaic-Christian faith fits that definition.
I'm keeping an open mind. After all, I do believe that MJ is a Judaism, so it's possible you can convince me. But know that I'm intellectually demanding -- you're going to have to come up with a REALLY good argument.
For example, here is my own argument that MJ is a Judaism. It is not a perfect argument, but I'd say it's a far better argument than the Reform Jews have.
- Maimonides gives the 13 principals of Judaism, and MJ fits it. There is a question over the ECHAD of Hashem, but for me, the fact that Trinitarianism CLEARLY states that God is ONE makes it fall within ECHAD. There is another question regarding corporality, as the Son became incarnate. However, that is only the Son; the Father and Spirit did not. It is obvious that the non-corporeal article is referring to the Father.
- A major feature of Judaism is that the Law cannot be understood without the Talmud/Oral Law. Even Reform Jews, though they do not follow the Talmud the way the Orthodox do, have not thrown it away entirely, and consider it a sacred text, giving it great honor. Those forms of MJ which incorporate elements of the Talmud (such as lighting Shabbat candles, celebrating Chanukah, separating meat and dairy, etc.) are a Judaism. Those "sola scriptura" versions of MJ which do not, are not a Judaism, but are more of a form of Protestantism.
- Unlike Messianism in general, the purpose of MJ is for Jews to strengthen their Jewish identity through following the Torah. (I'm not knocking Messianism, I'm just making a distinction here.) If it were not for this focus, the adherents would simply attend regular churches. Having this focus, doing mitzvot, keeping the Shabbat, eating Kosher, keeping the Holy Days, basically, being orthoprax, is the same emphasis as Judaism. As Mark Kinzer so aptly points out, It is Messianic that is the adjective and Judaism that is the noun. These are Jews doing Judaism who happen to believe in Mashiach--not really a whole lot different from the Chabadniks.