The Law of the Lord is perfect, reviving the soul

By the word perfect David means, that if a man is duly instructed in the law of God, he lacks nothing which is requisite to perfect wisdom. In the writings of the heathen authors there are no doubt to be found true and useful sentences scattered here and there; and it is also true, that God has put into the minds of men some knowledge of justice and uprightness; but in consequence of the corruption of our nature, the true light of the truth is not to be found among men were revelation is not enjoyed, but only certain mutilated principles which are involved in much obscurity and doubt.

As a soul gives vigor and strength to the body, so the law in like manner is the life of the soul. In saying that the soul is restored, he has an allusion to the miserable state in which we are all born. There, no doubt, still survive in some small remains of the first creation; but as no part of our constitution is free from defilement and impurity, the condition of the soul thus corrupted and depraved differs little from death, and tends altogether to death. It is, therefore, necessary that God should employ the law as a remedy for restoring us to purity; not that the letter of the law can do this of itself, but because God employs the Word as an instrument for restoring our souls.

Taken from Heart Aflame: Daily Readings from Calvin on the Psalms, day 60.