Psalm 139:6 (ESV)

6 Such knowledge is too wonderful for me; it is high; I cannot attain it.

The author realizes that this knowledge of God is far more than he can understand. This is too wonderful for us people. We cannot grasp it. We cannot even begin to comprehend this.

It is exactly as when an angel once spoke to Manoah, when he heard that his barren wife would indeed bear a child, and the angel said, Why do you ask my name, seeing it is wonderful? (Judges 13:18). Job in the end reached the same conclusion: I know that you can do all things, and that no purpose of yours can be thwarted. Who is this that hides counsel without knowledge? Therefore I have uttered what I did not understand, things too wonderful for me, which I did not know (Job 42:2–3). Let us therefore bow deeply before this mighty God. Let us worship him in honour and entrust ourselves to his good care.