Among Jesus' most surprising moral teachings is his warning about anger:

You have heard that it was said to your ancestors, ‘You shall not kill; and whoever kills will be liable to judgment. But I say to you, whoever is angry with his brother will be liable to judgment, and whoever says to his brother, ‘Raqa,’ will be answerable to the Sanhedrin, and whoever says, ‘You fool,’ will be liable to fiery Gehenna (Matthew 5:21-22).

With strong words like these from the lips of Jesus, it would be easy to conclude that anger has absolutely no place in the Christian life, but Scripture elsewhere teaches that some anger is not sinful: “Be angry but do not sin; do not let the sun set on your anger, and do not leave room for the devil” (Ephesians 4:26-27). Jesus himself expressed anger at the sins of the Jewish leaders, even driving the moneychangers out of the Temple with a whip (John 2:13-17). Although anger is traditionally identified as one of the seven capital vices (a.k.a. the "seven deadly sins"), the great Doctor of the Church St. Thomas Aquinas clarified, “lack of the passion of anger is also a vice” (Summa Theologica II, IIae 158.8). In other words, sometimes a Christian should be angry - within limits.

Here are three simple questions we can ask ourselves to determine if our anger is righteous or sinful:

#1 - Do I desire justice or vengeance?

To desire justice is good, but to desire vengeance is not. When we are aware of an injustice, it is good and holy to desire that the injustice be corrected. If one person is mistreating another, for example, we should feel within us a “righteous anger,” a desire that justice be restored. As the Catechism points out, however, we should resist any desire for revenge:

By recalling the commandment, “You shall not kill,” our Lord asked for peace of heart and denounced murderous anger and hatred as immoral. Anger is a desire for revenge. “To desire vengeance in order to do evil to someone who should be punished is illicit,” but it is praiseworthy to impose restitution “to correct vices and maintain justice.” If anger reaches the point of a deliberate desire to kill or seriously wound a neighbor, it is gravely against charity; it is a mortal sin. The Lord says, “Everyone who is angry with his brother shall be liable to judgment.” (CCC 2302)

#2 - Am I controlling my anger, or is my anger controlling me?

Although anger can motivate us to seek justice, it can also cloud our judgment and weaken our self-control. St. Alphonsus Liguori compared anger to an interior fire, which both spurs us into action with its flames and obscures our vision with its smoke. In our anger, we often exaggerate the faults of others and downplay our own. When anger motivates us to correct injustice, we must be very careful not to become unjust ourselves. Rather than simply giving in to the emotion of anger, then, we ought to guard against any uncharitable or irrational thoughts or actions that may arise. St. Alphonsus argued:

The acts of a man under the influence of anger cannot be conformable to the divine justice, and consequently cannot be faultless. A man who does not restrain the impulse of anger easily falls into hatred toward the person who has been the occasion of his passion.... Let us implore the Lord to preserve us from yielding to any strong passion, and particularly to anger.... For he that submits to such a passion is exposed to great danger of falling into a grievous sin against God or his neighbor (Sermon XXXIV).

#3 - Am I angry at the sin or the sinner?

You've probably heard the saying, “Love the sinner. Hate the sin” - Even though the phrase is somewhat cliche, it contains a great deal of truth. Jesus said, “love your enemies, and pray for those who persecute you, that you may be children of your heavenly Father, for he makes his sun rise on the bad and the good, and causes rain to fall on the just and the unjust” (Matthew 5:44-45). We have an obligation to love - i.e., “to will the good of” - every person without exception.

Precisely because we want what is best for others, we ought to have a holy hatred for sin itself, because sin can keep us the greatest good that there is -  eternal life with God. Because sin is like a spiritual cancer that can kill the soul, we should hate, or be angry at, sin itself. “Let love be sincere; hate what is evil, hold on to what is good” (Romans 12:9). St. Alphonsus again provides helpful clarity:

It is one thing to be angry against a brother, and another to be displeased at the sin of a brother. To be angry against sin is not anger, but zeal; and therefore it is not only lawful, but it is sometimes a duty. But our anger must be accompanied with prudence, and must appear to be directed against sin, but not against the sinner; for, if the person whom we correct perceive that we speak through passion and hatred towards him, the correction will be unprofitable and even mischievous. To be angry, then, against a brother's sin is certainly lawful. “He,” says St. Augustine, “is not angry with a brother, who is angry against a brother's sin.” ... But to be angry against a brother on account of the sin which he has committed is not lawful; because, according to St. Augustine, we are not allowed to hate others for their vices (Sermon XXXIV).

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