John Calvin Commentary Psalms 139:15

"My frame was not hidden from thee, When I was made in secret, [And] curiously wrought in the lowest parts of the earth." — Psalms 139:15 (ASV)

My strength was not hidden from you. David now begins to prove that nothing is hidden from God from the way in which man is first formed, and points out God’s superiority to other artisans in this: while they must have their work set before their eyes before they can form it, he fashioned us in our mother’s womb.

It is of little importance whether we read my strength or my bone, though I prefer the latter reading. He next compares the mother’s womb to the lowest caverns or recesses of the earth. If an artisan intended to start a work in some dark cave where there was no light to assist him, how would he set his hand to it?

In what way would he proceed? And what kind of workmanship would it prove? But God makes the most perfect work of all in the dark, for he fashions man in the mother’s womb. The verb רקם, rakam, which means weave together, is employed to amplify and enhance what the Psalmist had just said.

David no doubt means figuratively to express the inconceivable skill that appears in the formation of the human body. When we examine it, even to the nails on our fingers, there is nothing that could be altered without noticeable inconvenience, as if something were disjointed or out of place. And what, then, if we were to enumerate the individual parts?

Where is the embroiderer who—with all his industry and ingenuity—could execute the hundredth part of this complicated and diversified structure? We should not then wonder if God, who formed man so perfectly in the womb, should have an exact knowledge of him after he is ushered into the world.