Psalms 50:19

Thou givest thy mouth to evil, and thy tongue frameth deceit.

The verse describes the wicked as deliberately giving the mouth over to evil and shaping the tongue to frame deceit. It belongs with Psalms 24 and 25 in revealing the greatness of God. In its immediate setting this evil speech includes speaking against one's own brother and slandering him.

In the same setting this speech stands linked with consenting with thieves and associating with adulterers, as part of sharing in wrongdoing. Such evil and deceitful speech acts out and reveals a corrupt heart and life. It shows that the speaker rejects God's instruction and does not live by the truth.

A profession of holiness does not prove a person right with God, nor does speaking about God's covenant or teaching the Sabbath. Words and deeds expose the mismatch between profession and character. Giving the mouth to evil and framing deceit with the tongue is working on Satan's side instead of taking a stand under the rule of Christ.

This course dishonors God. It brings reproach on His cause and gives its enemies grounds to criticize, and it encourages others to transgress. Those who follow that course incur fearful loss. At the last great day those given to anarchy, war, and strife will see how many souls were lost through their failure.

The warning applies specifically to hateful speech that injures others' reputations, magnifies their supposed wrongs, and spreads evil suspicions. Those guilty of such speech must confess their own sins. They must guard their lips against wicked words.