Be angry and do not sin; do not let the sun go down on the cause of your anger.

The first instruction (in the previous verse) is to choose truthfulness as opposed to lies; to speak the truth to one another.

The latter part comes directly from Zechariah:

Those are the very words of God through Zechariah, not human opinion, but the very words of God himself.

So we have the instruction to lay aside falsehood and a second instruction to speak the truth.

Lay aside falsehood is what I call the negative commandment, meaning what you are not doing; speak the truth is the positive commandment.

Some may be content not to lie, but they keep their mouths from speaking the truth, maybe because they don’t want to offend anyone.

This is in opposition to the scriptures, which say that we should confess our sins to one another and pray for one another so that we can be healed (James 5:16). And in opposition to the words of Jesus, who said that if your brother sins you should go to him one on one (Matthew 18:15).

There should be truth-speaking; people should freely speak truth to one another, not live with the guard up around one another. That is Christ’s expectation of His church.

Are there people in your life that you are constantly telling the truth to (not under compulsion, which would be contrary to the spirit of this commandment here)?

That is not mandated, but it sounds like something positive based on Paul's words in the previous verse. It is a commandment to tell the truth to one another, not just a good idea.

Speaking the truth to one another is counter to being isolated from one another, especially if we see speaking the truth to one another as a positive commandment, meaning a requirement, a demand, an instruction.

But Paul talked about false brethren who came in to spy on our freedom (Galatians 2:4). That is a call for discernment.

Now we come to this verse, which also contains both a positive and a negative commandment.

Be angry is the positive commandment, and do not sin is the negative commandment, and they are a pair, just as lay aside falsehood and speak the truth (from the last verse) are a pair.

That you have to tell the truth to one another becomes an incentive to do the right thing in my opinion.

It would be a sad situation if you were not laying aside falsehood but were still speaking truth; it would be a situation where good and bad water are coming from the same pipe (James 3:11). That would not be right.

So, we should also not see “Be angry” as standing on its own without being paired with “do not sin.” And this is also coming from a verse in the Old Testament.

So whatever “Be angry” is, it is a stand-in for our emotional reaction to sin. In my estimation, it would be hard for us not to have a negative reaction to sin and not sin at the same time; a negative reaction to sin has to, by necessity, be paired with not sinning.

Anger is always a reaction. What Be angry is doing in that verse, when seen in the light of that Old Testament verse, is telling us about a negative reaction to sin, almost like an inward recoil that drives you to want to do something; hence, the “don’t let the Sun go down on the cause of your anger.”

Let me repeat: not sinning has an emotional quality. And the emotional quality of not sinning is what Paul is calling: “Be angry.”

That is not an encouragement to anger, given that Jesus said that being angry makes one a candidate for hell (Matthew 5:22).

It is not an encouragement to be angry, seeing that anger lies in the bosom of a fool (Ecclesiastes 7:9).

It is not saying to manage your anger, because we have a command not to be angry (Colossians 3:8).

I believe an interpretative problem arises when we see “Be angry” as separate from “Do not sin.”

The problem comes when we do not see that as our having a negative reaction to sin.

So the only way to interpret that verse in a way that ensures we do not wrongly endorse anger is to allow Psalm 4:4 to inform our understanding.

“Tremble with fear” was a commandment (Psalm 4:4), and Paul uses, I believe, a variation of it that says “be angry.” He possibly changed "tremble with fear” to “Be angry.”

We are emotional beings. We cannot have an egalitarian attitude toward sin; at a basic level, we need a negative reaction to it. We need to be revolted by sin at a basic level. Meaning we see things as God does, from the eye of His holiness.

Psalm 4:4 says, as you lie on your bed, think and repent. Paul said, "Do not let the sun go down on the cause of your anger," meaning that when you feel a sense of unease because of your sin, something is causing it; your conscience is stirred: repent.

“Do not let the sun go down” is a metaphor for as soon as possible, or as soon as it comes to your attention.

In the following verses, Paul mentions some sins we should “be angry” about and therefore avoid, and, if we do.

Do not let the sun go down on the cause of your anger; it can also mean repent. Take care of the personal issue of sin that is gnawing at you immediately; don’t postpone it.

That’s my interpretation of this verse.