John Calvin Commentary Psalms 139:6

"[Such] knowledge is too wonderful for me; It is high, I cannot attain unto it." — Psalms 139:6 (ASV)

Thy knowledge is wonderful above me. Two meanings may be attached to ממני: mimmenni. We may read upon me, or, in relation to me, and understand David to mean that God’s knowledge is seen to be wonderful in forming such a creature as man, who, to use an old saying, may be called a little world in himself. Nor can we think without astonishment of the consummate skill apparent in the structure of the human body, and of the excellent endowments with which the human soul is gifted.

But the context demands another interpretation; and we are to suppose that David, pursuing the same idea upon which he had already insisted, exclaims against the folly of measuring God’s knowledge by our own, when it rises prodigiously above us. Many, when they hear God spoken of, conceive of him as like themselves, and such presumption is most condemnable.

Very commonly, they will not allow his knowledge to be greater than what matches their own understanding of things. David, on the contrary, confesses it to be beyond his comprehension. He virtually declares that words cannot express this truth: the absoluteness with which all things lie open to the eye of God. This is a knowledge having neither bound nor measure, so that he could only contemplate its extent with conscious inadequacy.