David’s prayer of repentance in Psalm 51 draws on God’s self-description of his glory.

Exodus 34:6 וַיַּעֲבֹ֙ר יְהוָ֥ה׀ עַל־פָּנָיו֘ וַיִּקְרָא֒ יְהוָ֣ה׀ יְהוָ֔ה אֵ֥ל רַח֖וּם וְחַנּ֑וּן אֶ֥רֶךְ אַפַּ֖יִם וְרַב־חֶ֥סֶד וֶאֱמֶֽת׃

Exodus 34:6 “The LORD passed before him and proclaimed, “The LORD, the LORD, a God merciful and gracious, slow to anger, and abounding in steadfast love and faithfulness,”

Psalm 51:3 חָנֵּ֣נִי אֱלֹהִ֣ים כְּחַסְדֶּ֑ךָ כְּרֹ֥ב רַ֜חֲמֶ֗יךָ מְחֵ֣ה פְשָׁעָֽי׃

Psalm 51:1 “Have mercy on me, O God, according to your steadfast love; according to your abundant mercy blot out my transgressions.”

David’s plea for forgiveness is thus rooted in the firm foundation of God’s declaration of his glory.