When David sat down to write Psalm 139 and its exposition of divine omnipresence and omniscience, he was clearly overwhelmed by the magnificence of our Creator. It is important to note that David does not discuss these attributes as mere philosophical abstractions; rather, his discussion is the outflow of his love for and confidence in the Lord. Knowledge of God's character, in fact, gives him great assurance and motivates him to pray, trusting only in his Maker, which is evident in vv. 19–24 of this psalm.
Today's passage builds on David's discussion of divine omnipresence and omniscience in vv. 1–12. Verse 16 provides a reference to the Lord's comprehensive plan and decree by which He ordains and governs whatsoever comes to pass. David's days were written in God's book, the course of his life determined long before he was born. The author of this book is none other than the Lord Himself, for we see in other passages that God determines the precise number of years that a person will live. For example, when King Hezekiah prayed for healing, our Creator added fifteen years to his life, no more and no less (Isa. 38:1–5). This shows us that divine providence takes our actions into account, for God did not extend Hezekiah's life apart from his prayer. However, that does not mean that Hezekiah was the final determining agent. Hezekiah prayed because God ordained that he would pray, for God works out even the smallest details according to His plan (Eph. 1:11). The Lord is no passive observer who takes in knowledge like we do. He knows His creation because He has ordained everything in His creation. His sovereign will brings all things to pass.
Psalm 139:13–16 expands upon David's earlier references to divine presence, showing that the Lord's presence is close and intimate—so close, in fact, that God was there in the womb of David's mother knitting him together. The image pictures the Lord as attending to the details, just as a knitter has to pay close attention in order to knit a blanket and not produce a tangled mess. This passage also has important implications for how we view the unborn. If God knits people together in their mothers' wombs, we are forbidden from taking the life of the unborn for reasons of convenience.
This understanding of God's intimate, personal presence and knowledge motivates David to pray, as well it should (vv. 19–24). Such a being is worthy of our unquestioning trust and confidence. We know that such a being alone can help us.
Our knowledge of God's character not only encourages us to pray but it also helps keep us from sin. John Calvin writes, "One great reason of the carnal security into which we fall, is our not considering how singularly we were fashioned at first by our Divine Maker." When we remember how we were singularly made by the Lord, we are more aware that we are His possession, and therefore we are bound to do what is necessary so that we are holy as He is holy (Matt. 5:48).