If you are faithful to the Lord, this psalm will encourage you. If you are trying to hide from the Lord, this psalm will make you realize you are fighting a losing battle.
He knows what you do (1–6). He knows you personally and intimately, so do not try to fool Him. Be open and honest with God, and rest in His love (1 John 4:18). His eye is upon you, and you have nothing to fear.
He knows where you go (7–12). What a comfort to know that God is with you and cares for you! Whether you go up or down, east or west, He is there (Heb. 13:5). Sinners try to hide from God, but believers hide in God.
He knows what you are (13–16). After all, He made you, planned your potential, and ordered your days. This is not some kind of blind fatalism that paralyzes you. It is the wise plan of a loving Father who knows what is best for you. Accept what you are as His gift to you, and then use it wisely as your gift to Him. You are unique; God made you that way.
He knows what you think (17–18). The psalmist pondered the thoughts of God and in this way cultivated the presence of God in his life. “You are to meditate on these things” (Phil. 4:8).
He knows what you love (19–22). Take these words to heart: “You who love the Lord, hate evil” (Ps. 97:10); “Love not the world” (1 John 2:15–17).
He knows what you desire (23–24). You do not know your own heart as well as you may think you do (Jer. 17:9–10). Let God search you and deal with the things that make you anxious. Let Him lead you. He knows where you ought to go.
Wiersbe, W. W. (1997, c1991). With the word Bible commentary (Ps 139:1). Nashville: Thomas Nelson.