Have mercy upon me, O God, according to thy loving-kindness: according to the multitude of thy compassions put away mine iniquities.
Wash me thoroughly from mine iniquity, and cleanse me from mine sin.
For I know mine iniquities, and my sin is ever before me.
Against thee, against thee only have I sinned, and done evil in thy sight, that thou mayest be just when thou speakest, and pure when thou judgest.
Behold, I was born in iniquity, and in sin hath my mother conceived me.
Behold, thou lovest truth in the inward affections: therefore hast thou taught me wisdom in the secret of mine heart.
—Psalm 51:1-6 (1599 Geneva Version)

In David’s penitential pleadings, John Calvin (1509–1564) finds clear teaching on original sin. Calvin observes that contrary to Pelagians—who teach that people are born without sin, and that they can will, choose, and do good without the assistance of the Holy Spirit—David does not locate his fall from grace in the imitation of Adam’s sin. Instead, David faults his own participation in the same corrupted flesh.

The passage affords a striking testimony concerning original sin, which Adam brought on the whole human race. For this reason, it can also assist us in forming a correct description of it.

The Pelagians, to avoid what they considered the absurdity of holding that all were ruined through one man’s transgression, maintained that sin descended from Adam only through the imitation of others. But the Bible, both here and in other passages, clearly asserts that we are born in sin, and that it exists within us as a disease fixed in our nature.

David does not charge it on his parents or trace his crime to them but places himself before the judgment seat of God and confesses that he was formed in sin and that he was a transgressor before he emerged into the light.

Psalms 1-72, ed. Herman Selderhuis, Reformation Commentary on Scripture, OT Vol. VII, p. 382.