John Gills Exposition of the Bible Commentary - Psalms 139:13
For thou hast possessed my reins ,.... His thoughts and counsels, the reins being the seat of instruction and counsel; hence God is called the trier of the reins, and searcher of the hearts of the children of men; he is the possessor or master of their most secret thoughts, and thoroughly knows them; see Psalm 7:9 ; they are also the seat of the affections, which are naturally sinful and inordinate, and set upon carnal and earthly things; but the Lord possesses and engrosses the affections... read more
Matthew Henry's Complete Commentary - Psalms 139:7-16
It is of great use to us to know the certainty of the things wherein we have been instructed, that we may not only believe them, but be able to tell why we believe them, and to give a reason of the hope that is in us. David is sure that God perfectly knows him and all his ways, I. Because he is always under his eye. If God is omnipresent, he must needs be omniscient; but he is omnipresent; this supposes the infinite and immensity of his being, from which follows the ubiquity of his presence;... read more