Psalm 51, commonly known as the Miserere (from its Latin opening words, Miserere mei, Deus – “Have mercy on me, O God”), is one of the most profound penitential psalms in the Bible. The provided text is a responsorial form used in liturgy, with the refrain “Have mercy on us, O Lord, for we have sinned” interspersed among selected verses.

Its origins trace back to ancient Israel. Traditionally attributed to King David, the psalm's superscription links it to the prophet Nathan's confrontation after David's adultery with Bathsheba and the murder of Uriah (2 Samuel 11–12), around 1000 BCE. This superscription may reflect later editorial addition, with some scholars suggesting a pre-exilic or even post-exilic composition due to linguistic features. Regardless, it expresses deep contrition, acknowledgment of sin against God alone, and a plea for inner renewal – “Create in me a clean heart, O God” – making it a timeless expression of repentance.

In Christian tradition, Psalm 51 holds immense significance, classified as one of the seven Penitential Psalms. It features prominently in liturgy, including the start of the Liturgy of the Hours (“O Lord, open my lips...”) and famous musical settings, such as Gregorio Allegri's polyphonic Miserere (c. 1630s), composed for the Sistine Chapel.

In the liturgy, Psalm 51 is especially associated with Ash Wednesday, the solemn start of Lent. The Responsorial Psalm for Ash Wednesday Mass draws directly from it (verses 3–4, 5–6ab, 12–13, 14, 17), with the response “Be merciful, O Lord, for we have sinned” (or similar). This aligns perfectly with the day's themes of repentance, humility, and mortality, emphasised by the imposition of ashes with the words “Repent, and believe in the Gospel” or “Remember you are dust, and to dust you shall return.” The psalm's pleas for cleansing and restoration echo the Lenten call to conversion, preparing the faithful for Easter through fasting, prayer, and almsgiving.

Its use underscores God's boundless mercy amid human frailty, inviting believers to begin Lent in humble acknowledgment of sin and hopeful trust in divine compassion.