Psalm 139 is a short course in theology, the science of God. In the first six verses the psalmist says, "God knows everything; don't try to fool Him."
"O Lord, You have searched me and known me" (v. 1). In these first six verses we find 13 personal pronouns. God knows us personally. Few people can recognize us in a crowd, but God does. With Him there are no crowds, only individuals. At times you may feel lonely and say, "Nobody knows me. Nobody cares about me." But God knows you intimately. He knows your every action and thought. "You know my sitting down and my rising up; You understand my thought afar off" (v. 2). He also knows your words. "For there is not a word on my tongue, but behold, O Lord, you know it altogether" (v. 4). The psalmist is saying, "He's behind me. He's before me. He's laid His hand upon me. He is sovereign."
What should be our response to this? Simple. "Praise the Lord!" I am glad that my Father in heaven understands me personally and intimately and that His hand is upon me. This doesn't make me afraid; it gives me confidence. What an encouragement to know that our Father in heaven knows all about us--where we are and what we're doing. Of course, we don't want to be in the wrong place, doing the wrong thing. But if the child of God is walking in the will of God, he has the confidence that His Father in heaven is caring for Him and knows his every need.
God has a thorough knowledge of you. That gives you all the more reason to pray honestly to the Lord and walk uprightly before Him. His knowledge of you ought to encourage you and make you confident. Thank Him that His intimate knowledge of you leads to His complete care for you.