Great questions!
Let’s start by looking at the context of the Psalm. The note at the beginning of Psalm 51 says, “a psalm of David, when Nathan the prophet came to him after he had gone in to Bathsheba.” Do you recall the context? Take a look at 2 Samuel 12. This is the narrative when David, having committed adultery with Bathsheba and getting her pregnant, also had her husband killed in battle so that he could take her (without making it public that he had sinned). Nathan comes and calls David out on it.
In 2 Sam 12:13, we see David’s response: “I have sinned against the Lord.” Psalm 51 is David’s lament and prayer of contrition. So, let’s see what David is saying…
Psalm 50 (51): “A sacrifice to God is an afflicted spirit: a contrite and humbled heart, O God, thou wilt not despise”.
The translations I’m accustomed to reading render this slightly differently:
My sacrifice, O God, is a contrite spirit;
a contrite, humbled heart, O God, you will not scorn.
It’s not just that “a sacrifice to God is an afflicted spirit”; rather, David is saying that, at this point, he cannot offer an animal sacrifice, because of his sinfulness. Therefore, the sacrifice
he offers is his contrite spirit!
Contrite means crushed by one’s sins?
No; it means that he’s sorry for his sins.
St Thomas Aquinas told us we learn by our senses. Let us therefore use them.
What image can we use when explaining “humble”?
Not “humble”, but “humbled.” I think that the distinction is important. It’s not that David is a humble man, it’s that this public revealing of his sins has been a humbling experience for him. Therefore, he’s reaching out to God, and pleading with Him, reminding Him that He does not reject those who come to him with sorrow and who recognize the impact of their sins.
God has been explained by using an image of the Trinity. How would one explain the “God despises”?
The word I’m used to seeing is “scorn.” The image is one that we see throughout the OT: God refuses the ritual sacrifices of those who bring them without sincerity. Take a look at the psalm that precedes this one. In it, we see God speaking to his people (Psalm 50:7-9, 14):
“Listen, my people, I will speak;
Israel, I will testify against you; God, your God, am I.
Not for your sacrifices do I rebuke you, your burnt offerings are always before me.
I will not take a bullock from your house, or he-goats from your folds.
…[o]ffer praise as your sacrifice to God."
David
knows that God doesn’t want sacrifice without contrition; and so, he pleads his case that he
is sorry for his sins.
The image here, I think, is one of a child appealing to his father who despises insincere expressions of ‘love’ but who approves sincere ones.
