Be angry and do not sin; do not let the sun go down on your anger, – Ephesians 4:26
In Jesus we have a new existence. Right before he writes about anger to his friends in Ephesus, Paul has been explaining about putting on a whole new life and taking off the old one. They need to see that in Jesus they are completely new creatures made in God's image. So Paul gives them some practical examples of what these new creatures made in God's image can look like. This is an example of what it would be like to live imitating God's love.
Do you ever get worked up over an imaginary conversations with someone you're upset with? Or rehash old disagreements and hurts? Your heart races, your blood pressure goes up, and your feelings about that person are actually altered. There are physical responses in the body from a conflict that isn't happening!
When we harbor anger, we give the devil an opportunity to advance his strategies by sowing fear and descension. He loves to destroy unity. We don't make great decisions when we're angry. Paul is warning us that when we get angry there is a real chance to do real harm. There are situations that deserve our anger, but even in those situations we are liable for our actions. "I was just so angry" isn't a valid excuse for sin, for using our words to wound, to be unforgiving. Even if they are wrong, you will answer for your words and actions. There are consequences. Anger is not a good reason to write that post, fire off that comment, or passive aggressively link to that article. Real harm is done. The devil advances his purposes this way, through our actions.
We are locked in a spiritual battle, and your adversary is cunning and looking for every opportunity to destroy you and anything good. He only needs a small window, like your righteous (or unrighteous) anger. Instead of taking out your anger on your steering wheel or taking it online, take it to God. Explain it to him in his presence. See what he would have you do with it (Hint: it will be loving and building up and difficult, but he's promised to help you so go to him!).
Memorize this with me. Live this with me. Help me with this.