The following is an excerpt from The MacArthur New Testament Commentary on Ephesians 4.

Be angry, and yet do not sin; do not let the sun go down on your anger, and do not give the devil an opportunity. (Ephesians 4:26–27)

Parorgismos (anger) is not momentary outward, boiling–over rage or inward, seething resentment, but rather a deep–seated, determined and settled conviction. As seen in this passage, its New Testament use can represent an emotion good or bad, depending on motive and purpose.

Paul’s command is to be angry (from orgizo), with the qualification and yet do not sin. In this statement he may be legitimating righteous indignation, anger at evil, at that which is done against the Person of the Lord and against His will and purpose. It is the anger of the Lord’s people who hate evil (Ps. 69:9). It is the anger that abhors injustice, immorality, and ungodliness of every sort. It is the anger of which the great English preacher E W. Robertson wrote in one of his letters. When he once met a certain man who was trying to lure a young girl into prostitution, he became so angry that he bit his lip until it bled.

Jesus expressed righteous anger at the hard–heartedness of the Pharisees who resented His healing the man with the withered hand on the Sabbath (Mark 3:5). Although the word itself is not used in the gospel accounts of the events, it was no doubt that kind of anger that caused Jesus to drive the moneychangers out of the Temple (Matt. 21:12; John 2:15). Jesus was always angered when the Father was maligned or when others were mistreated, but He was never selfishly angry at what was done against Him. That is the measure of righteous anger.

Anger that is sin, on the other hand, is anger that is self–defensive and self–serving, that is resentful of what is done against oneself. It is the anger that leads to murder and to God’s judgment (Matt. 5:21–22).

Anger that is selfish, undisciplined, and vindictive is sinful and has no place even temporarily in the Christian life. But anger that is unselfish and is based on love for God and concern for others not only is permissible but commanded. Genuine love cannot help being angered at that which injures the object of that love.

But even righteous anger can easily turn to bitterness, resentment, and self–righteousness. Consequently, Paul goes on to say, do not let the sun go down on your anger, and do not give the devil an opportunity. Even the best motivated anger can sour, and we are therefore to put it aside at the end of the day. Taken to bed, it is likely to give the devil an opportunity to use it for his purposes. If anger is prolonged, one may begin to seek vengeance and thereby violate the principle taught in Romans 12:17–21,

Never pay back evil for evil to anyone. Respect what is right in the sight of all men. If possible, so far as it depends on you, be at peace with all men. Never take your own revenge, beloved, but leave room for the wrath of God, for it is written, “Vengeance is Mine, I will repay,” says the Lord. “But if your enemy is hungry, feed him, and if he is thirsty, give him a drink; for in so doing you will heap burning coals upon his head. Do not be overcome by evil, but overcome evil with good.”

It may also be that verses 26b–27 refer entirely to this unrighteous anger, in which case Paul uses the imperative in the sense of saying that, because anger may come in a moment and overtake a believer, and because it has such a strong tendency to grow and fester, it should be dealt with immediately—confessed, forsaken, and given to God for cleansing before we end the day.

In any case of anger, whether legitimate or not, if it is courted, “advantage [will] be taken of us by Satan” (2 Cor. 2:11), and he will feed our anger with self–pity, pride, self–righteousness, vengeance, defense of our rights, and every other sort of selfish sin and violation of God’s holy will.