Medieval tradition ascribed the authorship of the Seven Penitential Psalms (6, 31, 37, 50, 101, 129, and 142) to King David, who composed them as penance for his grievous sins. These transgressions included adultery with Bathsheba and the murder of her husband, Uriah (David had the unsuspecting spouse sent to the front lines of battle, insuring his death). The prophet Nathan reproached the king, and in spite of David's repentance and forgiveness by God, his son was taken from him. David repented more and was forgiven. In a second occurrence of sin, David offends God out of pride by commanding a census of Israel and Judah. This time the prophet Gad rebukes the ruler, and God sends, as punishment, a choice of famine, war, or pestilence. After plague ravages Israel, David's penance appeases the avenging God.
These particular seven Psalms have a long history associated with atonement. It is thought that by the third century, and probably much earlier, they had formed a part of Jewish liturgy. In the Christian tradition, they were certainly known by the sixth century, when the Roman author and monk Cassiodorus referred to them as a sevenfold means of obtaining forgiveness. Since the number of these Psalms was the same as the Deadly Sins, the two became linked, and the Penitential Psalms were recited to ask for forgiveness for the dead. The Psalms were thought especially efficacious in reducing the time the departed had to spend in purgatory. But it is also clear that the Psalms were recited to benefit the living, as a means of avoiding these sins in the first place. This was important because the Seven Deadly Sins--pride, covetousness, lust, envy, gluttony, anger, and sloth--had the ability to land one in hell for all time. This is why they were called Deadly or Mortal.
The Penitential Psalms usually follow the Hours of the Cross and the Hours of the Holy Spirit, beginning with Psalm 6, “Domine, ne in furore tuo arguas me...” (O Lord, rebuke me not in thine anger). They are followed immediately by the Litany, a hypnotic enumeration of saints whom one asked to pray for us. The list begins with Kyrie eleison, Christe eleison, Kyrie eleison (Lord, have mercy; Christ, have mercy; Lord, have mercy); God the Father, the Holy Spirit, and the Trinity are then invoked. Following these preliminary petitions is the Litany proper. It is a list of saints with each invocation followed by Ora [orate in the plural] pro nobis (Pray for us). The Virgin heads the list, followed by archangels, angels, other celestial spirits, and John the Baptist (our future intercessor at the Last Judgment). Next are the apostles, male martyrs, confessors (male non-martyr saints), female virgin martyrs, and, finally, widows. These ranked categories reflect the hierarchy not only of heaven but also of medieval society.
After these enumerations comes a recitation of the Agnus Dei (Lamb of God) and a repetition of the shortened Kyrie (both extracts from the Mass). The Litany concludes with various prayers for the dead, of which the characteristic Fidelium Deus is always present.