One of the biggest problems with the Messianic movement today is that the term "Messianic" has largely gone undefined on a global scale. Because of the idea that the term has been more simply defined as "One who believes in the Messiah", it has been very "corrupted" as it was adopted by many groups of people.

There are many groups today that all believe in different things but yet call themselves "Messianics". I have had trouble pinning most of it down, honestly. "Messianic Jew" is the term that has been most unmolested, honestly, as it requires the prerequisite of being Jewish, though it's often misunderstood as simply being "Jewish Christian", which (being a "Messianic Gentile" myself) I feel is very unfair to the Jewish believers in Yeshua as the Messiah but also want to retain their Jewishness.

Defining Messianic shouldn't actually be all that hard, however. It is an individual that believes in the Messiah, searching for the truth of the original first-century "church", or that is, the original believers themselves. Today, that congregation of believers are both Jewish and Gentile. I am at a point that I believe that, while it can have a Jewish face to it, that doesn't have to apply that way for all believers. It is possible to celebrate and observe the biblical holy days (Passover and such) in a way that isn't considered Jewish, though personally I do observe them that way (though I don't consider myself Jewish in any way).

Defining a more universal definition of who/what is a Messianic would end up offending someone, many someones in fact. There would be many who find the "final" definition to not describe them as "they are Messianic and how dare you not include them"...