Why this thread is retarded:
While Christianity started with Jews from Jerusalem and Judaism radically shifted due to Jews from Jerusalem it's the details that matter.
There were opposing groups, most notably the Pharisees who believed Moses was given an oral tradition at Sinai. This was opposed by Christ and the first Christians. The Pharisees had considerable control of the Sanhedrin and Herod had a lot of control in who was placed on the court, leading to further schism between them and the other groups that did not believe in their oral tradition. There is a clear enmity between these groups that eventual leads to the Pharisees making false claims that end in Christ crucified. The first Christians would teach that Christ was the Messiah and that he resurrected in the synagogues leading to further hostility from the Rabbis. Eventually, Christianity began to grow, at a time that the remaining Jews would experience the destruction of their homeland by the Romans. Out of fear of it being lost forever the remaining Pharisees wrote down their oral tradition and would eventually become the only group left still calling themselves Jews. This is why all contemporary Judaism is guided by the Talmud when the Jews in Jesus' time did not. The differences between the Talmud and the New Testament are profound and because of them Christians like Luther are astounded when they learn what contemporary Jews actually believe.
It's a very fallacious statement to call a Christian a Jew like this because the first Christians of Jewish lineage were thousands of years ago. The Jews of today have radically changed from how they were when Christianity started due to Rabbinical Judaism. It's more honest to say Jews were Christ's killers because all Jews today believe in the Pharisees' oral tradition while Christianity immediately moved away from Jewish customs.