St John Cassian (c. 360 - 435)
What are we to say of those whose anger is not bounded by “the going down of the sun” (Eph 4:26), but is prolonged for several days? They may say they are not angry, but in fact they show that they are extremely so. They do not speak pleasantly [to the person they are angry with], nor address them with ordinary civility; yet just because they are not actually seeking revenge, they think they are doing no wrong. But since they are unable to show their anger openly, they drive its poison into their hearts and secretly cherish it there…. Wrath that is nursed in the heart, although it may not injure others, excludes the splendid radiance of the Holy Spirit. 