Psalms 51:5 Judgment

One possible explanation of Psalms 51:5 is that the verse is speaking of David and Bathsheeba's son who died in judgment.

2Sa 12:13 And David said unto Nathan, I have sinned against the LORD. And Nathan said unto David, The LORD also hath put away thy sin; thou shalt not die.

2Sa 12:14 Howbeit, because by this deed thou hast given great occasion to the enemies of the LORD to blaspheme, the child also that is born unto thee shall surely die.

Notice  2 Samuel 12:13 says "I have sinned against the Lord" as does Psalms 51. Psalms 51 is a mirror and uses the same wording.

So that the judgment of taking away David's son was justified in God's sight.

Psa 51:4 Against thee, thee only, have I sinned, and done this evil in thy sight: that thou mightest be justified when thou speakest, and be clear when thou judgest.

Though David was forgiven, there were judgments that had taken place, David would lose his son.

Psa 51:5 Behold, I was shapen in iniquity; and in sin did my mother conceive me.

If v.5 is speaking of David's son, it would show that God wanted the lineage of Christ to be above board. If done in iniquity it was not of faith. The firstborn would always try to exalt himself over Solomon, and his lineage over Christ.

David's sin not only gave his current enemies ammunition, but could call into question the messianic lineage if the child were to live. There would always be the accusation that his son was Urriah's child.

The true lineage would be through Solomon.

David was under a stricter judgment for God had promised Solomon and later the messiah would come through him.

2Sa 7:12 And when thy days be fulfilled, and thou shalt sleep with thy fathers, I will set up thy seed after thee, which shall proceed out of thy bowels, and I will establish his kingdom.

2Sa 7:13 He shall build an house for my name, and I will stablish the throne of his kingdom for ever.

2Sa 7:14 I will be his father, and he shall be my son. If he commit iniquity, I will chasten him with the rod of men, and with the stripes of the children of men:

2Sa 7:15 But my mercy shall not depart away from him, as I took it from Saul, whom I put away before thee.

2Sa 7:16 And thine house and thy kingdom shall be established for ever before thee: thy throne shall be established for ever.

Psalms 51:5 may be a reference to David's son, and the sin of Bathsheba and David. "In sin did my mother conceive me."

In this case it wouldn't be a reference to original sin, but the sin of David and Bathsheba.