Charles H. Spurgeon on Psalms 139:13

For thou hast possessed my reins. Thou art the owner of my inmost parts and passions: not the indweller and observer only, but the acknowledged lord and possessor of my most secret self. The word "reins" signifies the kidneys, which by the Hebrews were supposed to be the seat of the desires and longings; but perhaps it indicates here the most hidden and vital portion of the man; this God doth not only inspect, and visit, but it is his own; he is as much at home there as a landlord on his own estate, or a proprietor in his own house. Thou hast covered me in my mother's womb. There I lay hidden —covered by thee. Before I could know thee, or aught else, thou hadst a care for me, and didst hide me away as a treasure till thou shouldest see fit to bring me to the light. Thus the Psalmist describes the intimacy which God had with him. In his most secret part —his reins, and in his most secret condition —yet unborn, he was under the control and guardianship of God.