As we rehearsed this composition, I noticed that the English words printed below the Latin text didn't seem quite accurate. The Latin text is written in the singular (as can be seen by the words mei and at the end, meam). Also, something about them seemed familiar. I decided to search for a parallel passage in the Bible. It didn't take long to find.
Below I present four versions of this text: the Latin which appears in the composition, a literal English translation, Ehret's version, and finally the Hebrew text itself:
Latin: Miserere mei, Deus,
English: Be gracious unto me, O God,
Ehret: Grant us mercy, O Lord,
Hebrew: Hhonenei Elokim
Latin: Secundum magnam misericordiam tuam,
English: According to Thy mercy,
Ehret: Now in thy goodness and lovingkindness
Latin: Et secundum multitudinem miserationum tuarum,
English: According to the multitude of Thy compassions,
Ehret: And according to the multitude of thy tender mercies
Hebrew: Kerov rahhamekha
Latin: Dele iniquitatem meam.
English: Blot out my transgressions.
Ehret: Forgive our iniquities we pray Thee.
Hebrew: Mehhe fesha'ai
The source is the Book of Psalms: Chapter 51, verse 3, and of particular interest, I think, are the two verses that precede it, (verses 1 and 2), and give us its context:
For the Leader, A Psalm of David; when Nathan the prophet came unto him, after he had gone in to Bath-sheba.
The context of this Psalm, then, is that it was written following the prophet Nathans rebuke of King David for his actions with respect to Bat-sheva.
The chapter continues with a prayer and confession by David:
Wash me thoroughly from mine iniquity,
And cleanse me from my sin.
For I know my transgressions;
And my sin is ever before me.
Against Thee, Thee only, have I sinned,
And done that which is evil in Thy sight;
That You may be justified when You speak,
And be in the right when You judge.
Hence it is clear that the Miserere Mei comes directly from the book of Psalms, and is yet another example of the Jewish historical roots of the church liturgy.