Joel doesn't think that Jesus is a true rabbi for the simple reason that he doesn't act like one. For one thing, Jesus doesn't strictly adhere to the ritual hand washing that is an essential part of the Mosaic law. In fact, Jesus doesn't seem to adhere to any part of that law at all. That's why he's so distrusted by the religious elders at the Temple; he's supposed to be a Jew yet does not follow the law as a Jew is supposed to. In Hebrew, the word rabbi means "my teacher," that is, someone who teaches Judaism. Jesus claims to be a rabbi, or teacher, but his shocking disregard for Jewish law convinces Joel and many others that, whatever else he might be, Jesus is not a true rabbi as they understand it.