O Lord, You have searched me and known me.
 You know when I sit down and when I rise up;
You understand my thought from afar.
 You scrutinize my path and my lying down,
And are intimately acquainted with all my ways.
 Even before there is a word on my tongue,
Behold, O Lord, You know it all.
 You have enclosed me behind and before,
And laid Your hand upon me.
 Such knowledge is too wonderful for me;
It is too high, I cannot attain to it.
These first six verses of Psalm 139 are about the omniscience of God. Simply stated God knows everything! He is intimately acquainted with each one of us. There's nothing we can hide from Him. He knows us even better than we know ourselves.
I find this knowledge both comforting and unnerving. God's omniscience is unnerving because He knows everything! I cannot hide my thoughts from Him. He knows when I doubt Him. He knows when I lust. He knows when hatred fills my heart and I mentally plan acts of vengeance. He knows when I say one thing with my mouth and another thing in my heart. I'm laid bare before Him. God knows me thoroughly and completely.
God's omniscience is comforting because God loves me in spite of the fact He knows all my awful flaws. He's not surprised when I sin or fail Him. He knew what I was going to do long before I actually do it, yet He still loves me. How do I know that God still loves me? I look back at the sacrifice He made at Calvary's cross. God loves me so much He punished His Son for my sin so He could forgive me. He did that with full and complete knowledge of my utter sinfulness. As David said, "Such knowledge is too wonderful for me; It is too high, I cannot attain to it." But I thank God for it!