Psalm 51 begins with David asking God, “Wash me (כַּבְּסֵ֣נִי) thoroughly from my iniquity… Purge me (תְּחַטְּאֵ֣נִי) with hyssop, and I shall be clean (וְאֶטְהָ֑ר); wash me (תְּ֝כַבְּסֵ֗נִי), and I shall be whiter than snow” (51:2, 7). The specific Hebrew words used here are important, because they’re all taken from laws about ritual purity. The verb “to wash” is used to describe the cleansing of garments from ritual impurity in Leviticus 11-17; “to purge” is used in Leviticus 14 to describe priests cleansing a house from unclean leprosy; and of course “to be clean” is the very word used throughout Leviticus 11-22 (and almost the whole OT) to describe a state of being ritually clean.
Thus it should be clear that, whatever David’s talking about in this Psalm, it has something to do with the purity laws of Leviticus. Indeed, even the very event from David’s life that this Psalm is centered around, namely his adultery with Bathsheeba, specifically highlights ritual purity as well (2 Samuel 11:4 cf. Leviticus 15). And it’s with this in mind that we should approach an oft-debated passage from this Psalm:
Behold, I was brought forth in iniquity, and in sin did my mother conceive me. (Psalm 51:5)
If this Psalm is all about Levitical purity laws, then this phrase can only be referring to one thing: Leviticus 12 and 15. In these chapters, we learn that conceiving children makes both husband and wife ritually unclean, and mothers who give birth to male children are “unclean for seven days,” which is why infants were circumcised “on the eighth day”; by this sequence, the text implies that children too were contaminated by their mother’s uncleanness until they were purified in circumcision (or an extra week for girls; see also Luke 2:22-24).
Tying this back to the broader context of Psalm 51, it becomes clear why Christians have always read Psalm 51:5 as a reference to original sin. As many have noted, the ritual purity laws of Leviticus recapitulate the curses of the Fall, and so when David recounts how he has been in sin from the moment of his conception, he’s clearly expounding how his unworthiness to stand in God’s “presence” extends far beyond his personal sin of adultery; ultimately, it stems from the original sin he shares as a descendent of Adam.
This reading is then further bolstered by the fact that, in 2 Samuel 12:13-14, prophet Nathan tells David that because of his sin with Bathsheeba, “the son who was born to you will die.” This suggests that David’s son was contaminated by his father’s sin, such that he could lawfully be put to death by the Lord (cf. 1 Samuel 2:6), a perfect image of our relationship with our forefather Adam.