John Calvin Commentary Psalms 138:8

"Jehovah will perfect that which concerneth me: Thy lovingkindness, O Jehovah, [endureth] for ever; Forsake not the works of thine own hands." — Psalms 138:8 (ASV)

Jehovah will recompense me, etc. The uncertainty associated with the meaning of the verb גמר, gamar, creates uncertainty over the whole sentence. Sometimes it means to repay, and, in general, to bestow, as it is often applied to free gifts. Yet the context seems to require another sense. Since it is added as a reason that Jehovah’s mercy is everlasting, and that He will not forsake the works of His hands, the better sense seems to be — Jehovah will perform for me; that is, He will continue to show that He cares for my safety and will fully perfect what He has begun.

Having once been delivered by an act of divine mercy, he concludes that what had been done would be perfected, as God’s nature is unchangeable, and He cannot separate Himself from that goodness which belongs to Him. Undoubtedly, the way to maintain good hope in danger is to fix our eyes upon the divine goodness, on which our deliverance rests.

God is under no obligation on His part; it is solely of His own good pleasure that He promises to concern Himself with us. David concludes with the best reason, from the eternity of divine goodness, that the salvation granted him would be of no limited and merely fleeting nature.

David confirms this still further by what he adds: that it is impossible God should leave His work, as men may do, in an imperfect or unfinished state through weariness or disgust. This David is to be understood as asserting in the same sense in which Paul declares that "the gifts and calling of God are without repentance" (Romans 11:29).

Men may abandon a work for very slight reasons which they foolishly undertook from the first, and from which they may have been diverted through their inconstancy, or they may be forced to give up through inability what they undertook beyond their strength. But nothing of this kind can happen with God, and therefore, we have no reason to fear that our hopes will be disappointed in their course towards fulfillment.

Nothing but sin and ingratitude on our part interrupts the continued and unvarying course of divine goodness. What we firmly grasp by our faith, God will never take from us or allow to pass out of our hands.

When David declares that God perfects the salvation of His people, he does not do so to encourage laziness, but to strengthen his faith and stir himself to the exercise of prayer.

What is the cause of that anxiety and fear which the godly feel, if not the consciousness of their own weakness and entire dependence upon God? At the same time, they rely with full certainty upon the grace of God, "being confident," as Paul writes to the Philippians, "that He who has begun the good work will perform it until the day of Christ Jesus" (Philippians 1:6).

The practical use of this doctrine is to remember, when we fall or are inclined to waver in our minds, that since God has worked the beginning of our salvation in us, He will carry it forward to its completion. Accordingly, we should turn to prayer, so that we may not, through our own laziness, block our access to that continuous stream of divine goodness which flows from an inexhaustible fountain.