Why do 'christians' always weasel into 'It's a metaphor' on the mustard seed passage?

According to the Jewish Kabbalah and Zohar, the entire Bible is a metaphor for what is to happen within our minds in order to purify. Therefore, when the Christian says, "It's a metaphor," he does no wrong. When he says, "this is literal," he's lying because all the Bible is metaphor.