‘Be angry, and do not sin. Do not let the sun go down on your angry mood. Neither give place to the Devil.'
The first phrase is taken from the LXX of Psalms 4:5. It recognises that anger in itself is not necessarily wrong. Indeed it tells us that we need to be angry if the cause is good. But it is a command that when we are angry we ensure it is short lasting and does not make us do wrong. The man who is too angry is least likely to make the right decisions.
At times anger against sin and wrongdoing is justified, and must be approved of, but not if it results in our behaving wrongly and failing to reveal the love of Christ. Nor if it festers in our hearts and minds. What we call ‘righteous anger' is often extremely unrighteous and self-defensive, and can reveal that the old man is still very much alive. As Paul says, we must be very careful, for wrongful anger gives a foothold to the Devil. We must bolt the door against him, for, as a Spanish proverb says, ‘from the fast-bolted door the Devil turns away'.
We can compare incidents in the life of Moses. He had every right to be angry with the constant failure and unbelief of the children of Israel, but he had no right to break the tablets written on by the finger of God (Exodus 32:19) and even less right to strike the rock twice in anger. The first was forgiven but the second blighted his future (Numbers 20:11-12).