Clement of Rome

Let us cleave, therefore, to those who cultivate peace with godliness, and not to those who hypocritically profess to desire it. For [the Scripture] saith in a certain place, "This people honoureth Me with their lips, but their heart is far from Me." [Isaiah 29:13] And again: "They bless with their mouth, but curse with their heart." [Psalm 62:4] And again it saith, "They loved Him with their mouth, and lied to Him with their tongue; but their heart was not right with Him, neither were they faithful in His covenant." [Psalm 78:36-37] …

Clement's First Letter to the Corinthians, Chapter 15 ↗

Augustine of Hippo

"May the Lord destroy all deceitful lips" [Psalm 12:3]. He says "all," that no one may suppose himself excepted: as the Apostle says, "Upon every soul of man that does evil, of the Jew first, and of the Greek." [Romans 2:9] "The tongue speaking great things:" the proud tongue.

Exposition on Psalm 12 ↗

Thomas Aquinas

"May the Lord destroy." Here he asks for their destruction. And first he asks for it; second, he suggests the cause, at "all deceitful lips," etc. -- as though he would destroy them twice, namely in soul and body. Jer. 17: "Bring upon them the day of affliction, and with a double destruction destroy them, O Lord God." Then he sets forth the cause of their malice, and he mentions three things: namely, fraudulence, because of "deceitful lips" -- for deceit is when one does one thing and pretends another. …

Exposition on the Psalms of David ↗