Ps 51:5 Surely I was sinful at birth, sinful from the time my mother conceived me.
This one verse takes a lot of explaining, but it also has to be consistent with all these verse
A Child is Innocent
Spiritual consequences of sin cannot be transmitted from father to son but only falls on the one who committed the act: Ezek 18:1-4; 18-20; Jer 32:29-30
Sin is committed by individually breaking God's law: 1 Jn 3:4
The spoken and written gospel message is God's power for salvation: Rom 1:16; 1 Cor. 1:18
God said that the king of Tyrus was "blameless in your ways from the day you were created, until unrighteousness was found in you." Ezek 28:15
"God made men upright but they sought devices" Eccl 7:29 (plural can't refer only to Adam)
Jer 19:2-6 human sacrifices of children to Baal is called the "blood of the innocent"
Jesus teaches us that we must become as little children to enter the kingdom of God (Matt. 18:3- 4; Lk. 18:16-17)
Apostle Paul: Rom 7:9-11 "Once alive" "sin killed me"
OK the verse in question:
Ps. 51: 5 Surely I was sinful at birth,
sinful from the time my mother conceived me.
Looking Deeper into Psalms 51:5
This is a Hebrew poetic parallelism, with the second line of the verse saying the same thing as the first line in a slightly different way. The first verb, of which David is the subject, is in the Pulal tense (as is "made" in # Job 15:7 ), which is an idiom used to refer to creation or origins, and is the 'passive' form of Polel ("formed": # Ps 90:2 Pro 26:10 ). TWOT, #623, 1:270.
The subject is, as the verse clearly states, the 'circumstances' of his conception- the sexual union which produced him was an act of sin, and
addresses the unrighteousness of his mother's act.
Looking at David’s Mother
David had two half-sisters (Zeruiah, Abigail)…..:
1CHR 2:13-16 13 “And Jesse begat his firstborn Eliab, and Abinadab the second, and Shimma the third, 14 Nethaneel the fourth, Raddai the fifth, 15 Ozem the sixth, David the seventh: 16 Whose sisters were Zeruiah, and Abigail. And the sons of Zeruiah; Abishai, and Joab, and Asahel, three. 17 And Abigail bare Amasa: and the father of Amasa was Jether the Ishmeelite.”
….and the father of David’s half-sisters was not Jesse, but Nahash: 2Sam 17:25 “And Absalom made Amasa captain of the host instead of Joab: which Amasa was a man’s son, whose name was Ithra an Israelite, that went in to Abigail the daughter of Nahash, sister to Zeruiah Joab’s mother.”
Look at Jesse
Exodus 34:7 maintaining love to thousands, and forgiving wickedness, rebellion and sin. Yet he does not leave the guilty unpunished; he punishes the children and their children for the sin of the parents to the third and fourth generation.”
Matthew 1:5 Salmon the father of Boaz, whose mother was Rahab, Boaz the father of Obed, whose mother was Ruth, Obed the father of Jesse
Torah specifically forbids an Israelite to marry a Moabite convert, since this is the nation that cruelly refused the Jewish people passage through their land, or food and drink to purchase, when they wandered in the desert after being freed from Egypt.
Ruth is a Moabite and Jesse is a descendant less than four generations away.
There is lots of speculation among Jewish scholars but we know this about David from the Psalms:
King David to face such an intense ignominy, to be shunned by his own brothers in his home (“I have become a stranger to my brothers”), by the Torah sages who sat in judgment at the gates (“those who sit by the gate talk about me”) and by the drunkards on the street corners (“I am the taunt of drunkards”)? What had King David done to arouse such ire and contempt? And was there no one, at this time in his life, who would provide him with love, comfort and friendship?
The Psalms describe the life of a poor, despised and lowly individual, who lacks even a single friend to comfort him. It is the voice of a tormented soul who has experienced untold humiliation and disgrace. Through no apparent cause of his own, he is surrounded by enemies who wish to cut him down; even his own brothers are strangers to him, ravaging and reviling him.
David was a half brother to the others and his mother was previous married to a gentile and his father might have started having a problem with what he had done since he knew scripture.
You might look up what Jewish scholar say about Psalm 51 since they have a problem with it also, but all realize it is talking about the mother's sin at her conception being the problem and not David being a sinner.