John Calvin Commentary Psalms 40:11

"Withhold not thou thy tender mercies from me, O Jehovah; Let thy lovingkindness and thy truth continually preserve me." — Psalms 40:11 (ASV)

O thou Jehovah! withhold not thy tender mercies from me. We now see more clearly what I have just referred to: that David speaks of his own thankfulness, so that he might secure a continuation of God’s favor toward him; and that he opened his mouth in the praises of God, so that he might continue to acquire new favors, against which our perverse and ungrateful silence very often closes the gate.

We should, therefore, carefully observe the relation that the clause in which David affirms that he did not close his lips has to what follows, namely, that God on his part would not restrict or block the course of his tender mercies. For this teaches us that God would always be ready to relieve us by his goodness, or rather that it would flow down upon us as from a never-failing fountain, if our own ingratitude did not prevent or cut off its course.

The tender mercies of God, which he expresses by the word רחמיד, rachamecha, and of which he speaks here, differ little from his goodness. It was not, however, without cause that David chose to make this distinction. It could only be, first, because he was unable otherwise to satisfy himself in extolling the grace of God; and secondly, because it was requisite to show the source from which the mercy and goodness of God proceed when he is moved by compassion for our miseries to aid and help us.

Then he places his confidence for salvation in the goodness and faithfulness of God, for we must necessarily begin (as I have said a little before) with the free favor of God, so that his bounty may extend even to us. But as we are unable to discern that God is gracious to us until he grants us some assurance of his love, his constancy is, very appropriately, placed in connection with his truth in keeping his promises.