THE SIN
Speaking evil is to accuse or slander with malicious or evil intent. It is using words to condemn. But this is not just a sin of the lips, for it is primarily a sin of the heart — a heart that bears a grudge, is filled with a desire for vengeance, or is driven by envy and jealousy. Therefore, speaking evil is murder with words.

THE SEVERITY
Whilst we often think speech sins are minor, unlike “bigger” sins like adultery or murder, James makes it quite clear they are serious and severe sins too — for they break God’s family, make light of God’s royal Law, and are, in effect, acts of rebellion against God.

THE SIGNIFICANCE
So why this teaching? The purpose is to show that such patterns of speech may reveal an unregenerate heart — that, contrary to their claims, they do not practice true religion if they constantly speak evil against one another. The hope is that they will not continue to do so, but will bridle their tongue and use it for words of blessing — thereby evidencing that they are truly born again as God’s people. And for those who do not see this “sign” of salvation in you, may you repent and believe in Jesus to be saved, and He will give you a new heart.