If then we wish to receive the Lord's blessing we should restrain not only the outward expression of anger, but also angry thoughts.
More beneficial than controlling our tongue in a moment of anger and refraining from angry words is purifying our heart from rancor and not harboring malicious thoughts against our brethren. The Gospel teaches us to cut off the roots of our sins and not merely their fruits. When we have dug the root of anger out of our heart, we will no longer act with hatred or envy. 'Whoever hates his brother is a murderer' (1 John 3:15), for he kills him with the hatred in his mind.
The blood of a man who has been slain by the sword can be seen by men, but blood shed by the hatred in the mind is seen by God, who rewards each man with punishment or a crown not only for his acts but for his thoughts and intentions as well. As God Himself says through the Prophet: 'Behold, I am coming to reward them according to their actions and their thoughts' (cf. Ecclus. 35:19); and the Apostle says: 'And their thoughts accuse or else excuse them in the day when God shall judge the secrets of men' (Rom. 2:15-16).
The Lord Himself teaches us to put aside all anger when He says: 'Whoever is angry with his brother shall be in danger of judgment' (Matt. 5:22). This is the text of the best manuscripts; for it is clear from the purpose of Scripture in this context that the words 'without a cause' were added later.
The Lord's intention is that we should remove the root of anger, its spark, so to speak, in whatever way we can, and not keep even a single pretext for anger in our hearts. Otherwise we will be stirred to anger initially for what appears to be a good reason and then find that our incensive power is totally out of control.
St John Cassian