But Jesus Was Jewish
One of the arguments I often get as to why I should accept Christianity/Messianic "Judaism" is that because Jesus and his followers were Jewish, therefore Jesus and his followers beliefs were Jewish.
Let's think about this for a second. The worshipers of the Golden Calf were Jewish. They were even monotheistic, believing that the Golden Calf was the G-d who took them out of Egypt. But was worshiping the Golden Calf a form of Judaism because its followers were Jewish? The answer is an obvious no, as the Jewish people were punished for this heinous sin.
Throughout the Book of Prophets we see that many Jews worshiped the pagan god, Ba'al. So does that make the worship of Ba'al a form of Judaism? The Bible is clear that it is not. In post-Biblical times (at least post-Biblical for Jews), we see that there were many Hellenist Jews who supported the worship of the Greek gods and were even supportive of the placement of an idol of Zeus in the Holy Temple. Just think about this for a second....Jewish worshipers are worshiping at the holiest place of all of Judaism! Doesn't that make the worship of Zeus a Jewish activity? The holiday of Chanukah, when the Jews fought the Greeks and their Hellenist Jewish supporters, stands to testify against this.
So, we can logically see that just because Jews worshiped or even created a religion or belief, it does not make this belief or religion Jewish. Similarly, it does not matter if the founders of Christianity (and Messianic "Judaism" is an 1800's creation of Christianity, originally called "Hebrew Christianity") were Jewish or not. Jesus and his followers created a religion, Christianity, which is simply not Jewish.