History and theological purpose of the liturgical Sequence portion of the Catholic Requiem Mass?
The sequence as you mention is a traditional Latin hymn and traditional religious poetry at the same time. It has been in the Roman Missal at least since the 13th century, with minor variations here and there.
It does not replace the Credo as the Credo is resited, sung or chanted on more festive occasions and on Sundays.
The sequence you are referring to is Dies irae which is recited at funerals and the Feast of All Souls (November 2).
This Missal text of the sequence is found, with light verbal variations, in a thirteenth-century manuscript in the Biblioteca. Nazionale at Naples (cf. Haberl, Magister Choralis, Ratisbon, 1900, pp. 237-238). Father Eusebius Clop, O.F.M., in the "Revue du chant Grégorien" (November-December, 1907, p. 49) argues a date between 1253-1255 for the manuscript--a. Franciscan Missal whose calendar does not contain the name of St. Clare, who was canonized in 1255, and whose name would have been inserted if the manuscript were later date. The same writer would assign (pp. 48, 49) a still earlier date (1250) to a copy of the Dies Iræ inserted at the end of a so-called "Breviary of St. Clare" dating about 1228. Into his arguments it is not necessary to enter here; but it is important to notice that these dates are much anterior to the dates of the manuscripts which, until recently, hymnologists had cognizance of when they attempted to fix the probable authorship of the sequence. Thus Mone found none anterior to the fifteenth century; Chevalier mentions only a Magdeburg Missal of 1480 and a manuscript Franciscan Missal of 1477; the first edition of Julian's "Dictionary of Hymnology" (1892) declared the "oldest form known to the present time" to be found in a Dominican Missal "written at the end of the fourteenth century and apparently for use at Pisa"; Warren, in his "Dies Irae" (London, 1902, p. 5), knows no earlier manuscript The second edition of Julian (1907) mentions the Naples manuscript in its supplement (p. 1629), but not the "Breviary of St. Clare". Father Clop describes also a third contemporary manuscript (p. 49), Italian, like the others: "Toutes trois enfin appartenant également à la liturgie des Frères Mineurs". All this renders very probable the conjecture generally entertained by hymnologists, that the Dies Iræ was composed by a Franciscan in the thirteenth century.
Its authorship has been most generally ascribed to Thomas of Celano, the friend, fellow-friar, and biographer of St. Francis. Reasons for this particularity of ascription are given by Keyser (Beiträge zur Geschichte und Erklärung der alten Kirchenhymnen, Paderborn und Münster, 1886, II, 194-196 and 230-235); also by Duffield (Latin Hymn Writers and Their Hymns, New York, 1889, 245-247), an ardent champion of the ascription to Thomas; also in "The Dolphin" (Nov., 1904, 514-516) which corrects a fundamental error in one of Duffield's main arguments. Ten other names have been suggested by various writers as the probable author of the Dies Iræ: (1) St. Gregory the Great (d. 604); (2) St. Bernard of Clairvaux (d. 1153); (3) St. Bonaventure (d. 1274); (4) Cardinal Matthew d'Acquasparta (d. 1302); (5) Innocent III (d. 1216) (6) Thurstan, Archbishop of York (d. 1140); (7) Cardinal Latino Orsini, or Frangipani, a Dominican (d. 1296); (8) Humbert, a general of the Dominicans (d. 1277); (9) Agostino Biella, an Augustinian (d. 1491); (10) Felix Haemmerlein, a priest of Zurich (d. 1457). - Dies Iræ
The Roman Rite sequence is above all a religious hymn in an artist poetic form. The sequence adds a liturgical artistic flare to the liturgy of the Mass.
The Latin sequence has its beginnings, as an artistic form, in early Christian hymns such as the Vexilla Regis of Venantius Fortunatus. Venantius modified the classical metres based on syllable quantity to an accentual metre more easily suitable to be chanted to music in Christian worship. In the ninth century, Hrabanus Maurus also moved away from classical metres to produce Christian hymns such as Veni Creator Spiritus.
The name sequentia, on the other hand, came to be bestowed upon these hymns as a result of the works of Notker Balbulus, who popularized the genre in the ninth century by publishing a collection of sequentiae in his Liber Hymnorum. Since early sequences were written in rhythmical prose, they were also called proses.
Notker's texts were meant to be sung. In the Latin Mass of the Middle Ages, it became customary to prolong the last syllable of the Alleluia, while the deacon was ascending from the altar to the ambo, to sing or chant the Gospel. This prolonged melisma was called the jubilus, jubilatio, or laudes, because of its jubilant tone. It was also called sequentia, "sequence," because it followed (Latin: sequi) the Alleluia. Notker set words to this melisma in rhythmic prose for chanting as a trope. The name sequence thus came to be applied to these texts; and by extension, to hymns containing rhyme and accentual metre. A collection of sequences was called the Sequentiale.
Dies Irae is a Latin hymn or sequence prescribed for the Roman Catholic Requiem Mass (Mass for the Dead or Funeral Mass) as well as on the Feast of All Souls (November 2) until the liturgical reforms which followed the Second Vatican Council. It could be noted that there are more translations in English than in any other modern language of this hymn. By 1913, the English renderings numbered 234, according to the Catholic Encyclopedia's article on the subject.
Fish Eaters has this to say about King David and the Sibyls:
These women are often depicted in medieval dramas, Jesse Trees and Nativity scenes. One hears of the Sibyls in Catholic chant and hymns, too: on Christmas Eve, after Matins and before Mass, The Song of the Sibyl was sung all over Europe until the Council of Trent (now this custom, restored in some places in the 17th c., remains mostly in Spain). They are most famously mentioned in the "Dies Irae," sung at Masses for the dead. Its opening lines:
Dies irae, dies illa,
solvet saeculum in favilla,
teste David cum Sibylla.
That day of wrath,
that dreadful day,
shall heaven and earth in ashes lay,
as David and the Sybil say.
Who were these women whom Christians group with King David and the great Old Covenant Prophets?
Who Was the Corroborating Sibyl in Dies Irae?
We are startled to have the Sibyl brought in as a supporting witness: Teste David cum Sibylla. The Sibyl is a virginal prophetess, who is the medium and mouthpiece of the religious or inspired traditions of paganism. The Erythrean Sibyl and the Samian Sibyl are credited with prophecies concerning the Last Judgment and the end of the world. The reference to the pagan prophetess is likely prompted by the practice of Christian art, which since the thirteenth century has placed the Sibyls at the side of, or rather opposite to, the prophets. We know today that the twelve books of Sibylline prophecy that have come down to us are not genuine, but among Christians of earlier centuries there was a widespread belief in their authenticity. St. Jerome has a passage that implies some credence on his part. Lactantius even quotes the pronouncements of the Sibyls as divinely inspired. The great St. Thomas declares: "Etiam Sibyllae multa vera praedixerunt de Christo" (Summa, II-II, Q. clxxii, art. 6). St. Augustine quotes a Sibylline utterance on the destruction of the world, but he seems to have his tongue in his cheek and says that the Sibylline prophecies in general can be looked upon as the inventions of certain Christians: "Istae prophetiae possunt putari a christianis esse confictae" (De Civ. Dei, lib. XVIII, cap. 47). Perhaps Thomas of Celano believed that certain individuals outside of the Chosen People received revelations concerning certain mysteries, and gave the Sibyl a distinction she did not deserve by adducing her as a witness. - Dies Irae, Masterpiece of Latin Poetry by Msgr. Paul E. Campbell, M.A., Litt.D., Ed.D.
As a side note, Michelangelo painted five Sibyls on the ceiling of the Sistine Chapel.
Let us remember that King David had incurred the wrath of God as expressed through the prophet Nathan (I Samuel 11-12).
Worst of all, David had one of his own soldiers killed so he could take his wife for himself, which incurred the wrath of God as expressed through the prophet Nathan (I Samuel 11-12). Yet David repented and sought forgiveness (Psalm 51 is associated with this incident), which God freely granted while not exempting him from the consequences of this flagrant infraction of his law. - A flawed king: David
It may be worth noting that Divine Worship (the Use of the Ordinariates) includes provision for the Dies Irae on All Souls' Day and in all Masses for the Dead, despite being a post-Conciliar liturgy of the Second Vatican Council. Other places may still have permission to use these tremendous sequences also.
sequence, but I don't think the Credo would be in a funeral mass for the sequence to replace it. The Credo is only said on Sunday Masses (and solemnities I think). And there are Sunday Masses (Pentecost for instance) that have a sequence.