John Calvin Commentary Psalms 86:15

"But thou, O Lord, art a God merciful and gracious, Slow to anger, and abundant in lovingkindness and truth." — Psalms 86:15 (ASV)

And you, O Lord, are God, merciful, ready to forgive. By immediately moving on to celebrate these divine attributes, he suggests that we find adequate strength and protection against the audacity and rage of the wicked in God's goodness, mercy, and faithfulness. Perhaps, also, because he felt that the wicked were scourges in God's hand, he focused on God's goodness and mercy to calm the excessive terror that might have seized him. For this is the true and only source of comfort: that although God chastises us, he does not forget his mercy.

This sentence, as is well known, is taken from Exodus 34:6, where we find a very remarkable description of God's nature. First, he is called merciful; next, ready to forgive, which he manifests by showing compassion for our distresses. Third, he is described as long-suffering, for he is not angry whenever an offense is committed against him, but pardons us according to the greatness of his loving-kindness.

In short, he is said to be abundant in mercy and truth; by which I understand that his beneficence is continually exercised, and that he is always true. He is indeed no less worthy to be praised on account of his rigor than on account of his mercy; but as it is our willful obstinacy alone which makes him severe, compelling him, as it were, to punish us, the Scriptures, in representing him as by nature merciful and ready to forgive, teach us that if he is at any time rigorous and severe, this is, as it were, accidental to him.

I am speaking, it is true, in popular language, and in language that is not strictly correct; but still, these terms by which the divine character is described amount in effect to this: that God is by nature so gracious and ready to forgive that he seems to connive at our sins, delays the infliction of punishment, and never proceeds to execute vengeance unless compelled by our obstinate wickedness.

Why the truth of God is joined with his mercy has been considered in another place. Since even the most generous people sometimes desire to retract the promises they have made, repenting of having made them so readily, we who are accustomed to unreasonably judge God by ourselves, distrust his promises. God therefore declares that he is unlike humans, because he is as firm in his purpose in abundantly performing whatever he has promised as he is distinguished for promising liberally.