What does Psalm 139:4 mean?

In this verse the psalmist, David, declares that the all-knowing God foreknows even the words he is about to speak. God knows what we are thinking and about to say. No wonder David declared at the end of the psalm: "Search me, O God, and know my heart! Try me and know my thoughts!" (Psalm 139:23).

What we say with our mouths results from the words we form in our minds, and those words form out of our thoughts. The fact that God knows the words we are about to speak should cause us to think carefully about our speech and our thoughts. Our words should be wholesome, healthful, and edifying. Colossians 4:6 exhorts: "Let your speech always be gracious, seasoned with salt, so that you may know how you ought to answer each person." Ephesians 4:25 says we should speak the truth with our neighbor. In the world we are surrounded by foul speech, lies, and words meant to tear down. We must resist the temptation to speak in kind. We should not engage in corrupt talk, but speak only words that are true and that build others up, "that it may give grace to those who hear" (Ephesians 4:29).