“The mouth of the righteous utters wisdom, and his tongue speaks justice. The law of his God is in his heart; his steps do not slip.” -Psalm 37:30–31
Jesus taught his disciples that we shall know someone by their fruits and that whatever comes out of the mouth is an indicator that reveals the condition of the speaker’s heart.
““Beware of false prophets, who come to you in sheep’s clothing but inwardly are ravenous wolves. You will recognize them by their fruits. Are grapes gathered from thornbushes, or figs from thistles? So, every healthy tree bears good fruit, but the diseased tree bears bad fruit. A healthy tree cannot bear bad fruit, nor can a diseased tree bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Thus you will recognize them by their fruits.” -Matthew 7:15–20
“The good person out of the good treasure of his heart produces good, and the evil person out of his evil treasure produces evil, for out of the abundance of the heart his mouth speaks.” -Luke 6:45
In our Psalm, David says the law of God is not only written on the righteous man’s heart as it is written on every man’s heart, but it is hidden there, stored up in the righteous man’s heart (Psalm 119:11). He keeps it there and meditates upon it day and night, and the fruit of his mouth reveals it because he only speaks wisdom and justice—and his walk is stable and constant because of it.