In Psalm 5, David says this:
"For there is no faithfulness in their mouth; their inward part is very wickedness; their throat is an open sepulchre; they flatter with their tongue." (Psa 5:9)
Notice here that there are people who can flatter you and seem great on the outside, but "their inward part is very wickedness". Contrast this with what God requires:
"Behold, thou desirest truth in the inward parts: and in the hidden part thou shalt make me to know wisdom." (Psa 51:6)
This second quote was from a Psalm that David wrote some time after his sin with Bathsheba, so David is not claiming to be a perfect example of virtue. What he is saying is that God wants us to be right from within, and no amount of flattery on the outside will cover a rotten inside. So also with us, we need to work on our own inner thoughts and desires first, so that the outward words and actions flow naturally from within.
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Rob de Jongh is the author of the book "Food for thought in Psalms" by Woodland Press