If you watched the old "Rocky and Bullwinkle" cartoon show, you probably recall a feature called "Mr. Know-It-All." Bullwinkle the Moose would give a short lecture on some "how-to" topic, and the information he gave was funny ... and usually just plain wrong.
Well, we know somebody who really does know it all, our heavenly Father. God knows everything. His knowledge is immediate, instantaneous, comprehensive, and fully retentive. God knows what He knows without any kind of painstaking research; He never had to go to school, or take a test, or be informed about anything. He never says, "Huh?" or "Oh, really?" You can never tell God something he doesn't already know. He knows it all.
In Psalm 139, David takes theology off the top shelf and brings it down to a personal level. To David, this truth is not theological or philosophical, it's relational and personal. Notice the personal pronouns here: "Lord, you have searched me and known me. You know my sitting down and my rising up. You understand my thought afar off ... " He doesn't say, "Lord, you know all things and you've searched all things," he says, "You know me, you've searched me."
I'm bringing this up because what David does in this psalm, we must do with all truth. We must not be content to leave it on a page of a book or discuss it with our friends over coffee: "What is the meaning of life?" "Who is God?" We have to personalize this, not just be content to be armchair theologians.
Let's take it further. "You understand my thoughts" (v. 2) could be expressed, "God, you know what I think before I even think it." What that means to us is this: God knows what you really believe about him, not just what you say. He knows where you stand, what your real opinions are.
When David says "Search me, O God, and know my heart" (v. 23), it's a prayer. He's inviting God to know more (even though that's not possible). He's saying, "God, I can't wrap my mind around this. I'll blow a fuse trying to figure this out, so I just surrender to it. Search me, know me, lead me, direct me."
And that's where we ought to leave off in dealing with God's omniscience, and all His divine attributes. I like to put it this way: Since God's ability transcends my reality, it's best for me to bow at His immensity. God is always greater than our present knowledge of Him. You see, if God was small enough for your brain, He wouldn't be big enough for your needs.
God indeed "knows it all." By His very nature, without having to learn anything, He already knows everything, past, present and future. Faced with that, what else can we do but bow to Him in worship and adoration?