Charles Ellicott Commentary Ephesians 4:26

"Be ye angry, and sin not: let not the sun go down upon your wrath:" — Ephesians 4:26 (ASV)

Be ye angry, and sin not.—A quotation from the Septuagint version of Psalm 4:4. Anger itself is not sin, for our Lord Himself felt it (Mark 3:5) at the “hardness of men’s hearts”; and it is again and again attributed to God Himself, in language no doubt of human accommodation, but, of course, accommodation to what is sinless in humanity. In the form of resentment, and above all, of the resentment of righteous indignation, it performs (as Butler has shown in his sermon on “Resentment”) a stimulating and inspiring function in the struggle against evil. But it is a dangerous and exceptional weapon; therefore, the exhortation “sin not”, and the practical enforcement of that exhortation in the next clause.

Let not the sun go down upon your wrath.—In this command (for which a Pythagorean parallel may be found), Saint Paul gives a twofold safeguard against the abuse of even righteous anger:

1. It is not to be prolonged beyond the sunset—beyond the sleep that ends the old day and leads in the freshness of the new, and which any godly person must prepare for by commending themselves to God and in prayer for His forgiveness, “as we forgive those who trespass against us.”
2. It is not to be brooded over and stimulated; for the word “wrath” is properly self-exasperation, being similar to the “contention” of Acts 15:30, described as alien to the spirit of love in 1 Corinthians 13:5.

It is that “nursing of wrath to keep it warm,” which can be checked even by those who cannot control the first outburst, and which constantly corrupts righteous indignation into selfish personal anger, if not into malignity.