Charles H. Spurgeon on Psalms 31:18

Let the lying lips be put to silence. A right good and Christian prayer; who but a bad man would give liars more license than need be? May God silence them either by leading them to repentance, by putting them to thorough shame, or by placing them in positions where what they may say will stand for nothing. Which speak grievous things proudly and contemptuously against the righteous. The sin of slanderers lies partly in the matter of their speech; "they speak grievous things; "things cutting deep into the feelings of good men, and wounding them sorely in that tender place —their reputations. The sin is further enhanced by the manner of their speech; they speak proudly and contemptuously; they talk as if they themselves were the cream of society, and the righteous the mere scum of vulgarity. Proud thoughts of self are generally attended by debasing estimates of others. The more room we take up ourselves, the less we can afford our neighbours. What wickedness it is that unworthy characters should always be the loudest in railing at good men! They have no power to appreciate moral worth of which they are utterly destitute, and yet they have the effrontery to mount the judgment seat, and judge the men compared with whom they are as so much chaff. Holy indignation may well prompt us to desire anything which may rid the world of such unbearable impertinence and detestable arrogance.

Verses 19-22. Being full of faith, the psalmist gives glory to God for the mercy which he is assured will be his position.