Paul gets pretty specific in that this walk entails—he talks about things like living in unity, speaking truthfully to each other, stealing from others and dealing constructively with our anger. Living (walking) like the people of God changes the way we respond to those things. And dealing with anger seems to be a particular issue for Paul. He tells not to sin in our anger but to rather process quickly, "Do not let the sun go down while you are still angry" (v. 26). He then warns that a failure to process our anger will give the devil a "foothold" or "opportunity" (ESV). The word here is literally "place." It’s the same word used for Jesus praying "at a certain place" (Luke 11:1). We give the devil a place in our lives when we allow anger to go unresolved. This must be important for us because Paul returns to anger later in the chapter (Eph 4:31-32)
31 Get rid of all bitterness, rage and anger, brawling and slander, along with every form of malice. 32 Be kind and compassionate to one another, forgiving each other, just as in Christ God forgave you.Bitterness, rage, brawling, slander and malice are all just different incarnations of anger. Brawling and rage happen when you act on anger immediately without thinking. Bitterness, slander and malice happen when we hold anger in and let it simmer in our hearts before it bubbles to the surface. We may think that rage is more honest as we blow up and then justify ourselves with, "Hey, I just say what I think." Paul would respond, "That’s fine, now repent don't do it again." Bitterness is what happens when we fail to work through (and pray through) our anger so that we can release it (even if the other person does not properly repent). Bitterness is a very dangerous thing; it twists us spiritually and emotionally.
And bitterness can also twist us up physically. Researchers from Concordia University in Montreal report that constant bitterness can lead to physical illness, affecting everything from organ function to immune response and vulnerability to disease. In other words, bitterness will make you sick. Researchers Carsten Wrosch and Jesse Renaud have written a book based on their studies entitled Embitterment: Societal, Psychological and Clinical Perspectives. There study suggests that bitterness affects many different systems in the body that prevent them from working as they should. Bitterness causes people to blame others for their problems rather than looking side themselves, and this process over time litter makes them sick.
Paul’s advice may help us physically as well as spiritually, "Don’t let the sun go down while you are still angry.” When an offense is serious enough to rise to the level of sin, then Jesus’ process in Matthew 18 ceases to be optional— we must go to the person directly (without talking to others) to resolve the issue in private. If that doesn't solve things, then we go back and take someone with you as a mediator. If that doesn’t work, then involve the church to mediate the issues. What we can’t do is allow things from personals slights to actual sins to fester and grow in our hearts and turn us into bitter people. When that happens, we are crippled spiritually and give the devil a place in our lives