The following is an excerpt from The MacArthur New Testament Commentary on Matthew 5.

and whoever shall say, “You fool,” shall be guilty enough to go into the fiery hell. (5:22c)

Moros (fool) means “stupid” or “dull” and is the term from which we get moron. It was sometimes used in secular Greek literature of an obstinate, godless person. It was also possibly related to the Hebrew marâ which means “to rebel against.” To call someone You fool was to accuse them of being both stupid and godless.

The three illustrations in this verse show increasing degrees of seriousness. To be angry is the basic evil behind murder; to slander a person with a term such as Raca is even more serious, because it gives expression to that anger; and to condemn a person’s character by calling him a fool is more slanderous still.

The Psalms twice tell us that “the fool has said in his heart, ‘There is no God”’ (Ps. 14:1; 53:1; cf. 10:4). The book of Proverbs is filled with references and warnings to fools. On the road to Emmaus Jesus used a similar, but less severe, term when He called the two disciples “foolish men and slow of heart to believe in all that the prophets have spoken!” (Luke 24:25).

Because of the testimony of God’s Word, we know that fools of the worst sort do exist. And it is our obligation to warn those who are clearly in opposition to God’s will that they are living foolishly. We certainly are not wrong to show someone what Scripture says about a person who rejects God. Jesus’ prohibition is against slanderously calling a person a fool out of anger and hatred. Such an expression of malicious animosity is tantamount to murder and makes us guilty enough to go into the fiery hell.

Geenna (hell) is derived from Hinnom, the name of a valley just southwest of Jerusalem used as the city dump. It was a forbidding place where trash was continually burned and where the fire, smoke, and stench never ceased. The location was originally desecrated by King Ahaz when “he burned incense in the valley of Ben-hinnom, and burned his sons in fire, according to the abominations of the nations whom the Lord had driven out before the sons of Israel” (2 Chron. 28:3). That wicked king had used the valley to erect an altar to the pagan god Molech, an altar on which one’s own children sometimes were offered by being burned alive. It would later be called “the valley of Slaughter” (Jer. 19:6). As part of his godly reforms, King Josiah tore down all the altars there and turned the valley into the garbage incinerator it continued to be until New Testament times. The name of the valley therefore came to be a metonym for the place of eternal torment, and was so used by Jesus eleven times.

To call a person a fool is the same as cursing him and murdering him, and to be guilty of that sin is to be worthy of the eternal punishment of fiery hell.