Among the difficult sayings of Jesus, Mark 9:49 is one of the most enigmatic. Almost all previous explanations of this verse have dealt with the Greek text, but like many of the difficult sayings of Jesus, this one simply cannot be explained from the Greek alone.
The original understanding of the third commandment, “You shall not take the name of the LORD your God in vain” (Exod. 20:7), was that one must keep one’s vows when swearing by God’s name. Israel’s ancient sages eventually came to interpret this commandment to mean using the LORD’s name lightly or frivolously. To avoid the risk of employing the divine name irreverently, the sages ruled that one should not utter it at all.