The Confiteor (so named from its first word, or incipit, in Latin, meaning "I confess" or "I acknowledge") is one of the prayers that can be said during the Penitential Act at the beginning of Mass of the Roman Rite in the Catholic Church. It is also said in the Lutheran Church at the beginning of the Divine Service, and by some Anglo-catholic Anglicans before Mass. The prayer is started by the Priest and ended by the people.
While the original Eastern liturgies begin with a confession of sin made by the celebrant, the earliest records of the Roman Rite all describe the Mass as beginning at the Introit, but the celebrant may have used a Confiteor-like confession of sinfulness as one of the private prayers he said in the sacristy before he began Mass. Only in the 10th or 11th century is there any evidence of the preparation being made at the altar. 
Outside of Mass some prayers similar to the Confiteor appear earlier. The "Canonical Rule" of Chrodegang of Metz (d. 743) recommends: "First of all prostrate yourself humbly in the sight of God ... and pray Blessed Mary with the holy Apostles and Martyrs and Confessors to pray to the Lord for you." And Egbert of York (d. 766) gives a short form that is the germ of our present prayer: "Say to him to whom you wish to confess your sins: through my fault that I have sinned exceedingly in thought, word, and deed." In answer the confessor says almost exactly the Misereatur.
The Confiteor is first found quoted as part of the introduction of the Mass in Bernold of Constance (d. 1100). The Misereatur and Indulgentiam follow, the former slightly different, but the latter exactly as it was in the Tridentine Missal. The Tridentine form of the Confiteor is found in the 14th-century "Ordo Romanus XIV" with only a slight modification, and is found word for word in a decree of the Third Council of Ravenna (1314).
The form, and especially the list of saints invoked, varied considerably in the Middle Ages. The Carthusian, Carmelite, and Dominican Offices, whose Missals, having existed for more than 200 years before 1570, were still allowed, had forms of Confiteor that differed from that in the Tridentine Missal. These three forms were quite short, and contained only one "mea culpa"; the Dominicans invoked, besides the Blessed Virgin, St. Dominic. Moreover, some other orders had the privilege of adding the name of their founder after that of St. Paul. The Franciscans for instance, inserted the name of St. Francis, and many Benedictine houses added the name of their founder, St. Benedict. The local patron was inserted at the same place in a few local uses.
To what is here taken from the Catholic Encyclopedia one can add the text of an elaborate form of the Confiteor found in the Paenitentiale Vallicellanum II, which has been attributed to the 9th century:
Usage in Catholicism
The text of the Confiteor in the 1970 Missal is as follows:
The form in the Tridentine Roman Missal (in Latin) is longer and is said twice, first by the priest in the following form, then by the altar server, who replaces the words "et vobis, fratres", "et vos, fratres" (and you, brethren) with "et tibi, pater" and "et te, pater" (and you, Father).
In the Tridentine editions of the Roman Missals, if a priest celebrated Mass in the presence of the Pope or a cardinal, or of a nuncio, a patriarch, a metropolitan archbishop or a diocesan bishop within their own jurisdictions, he changed "et vobis, fratres", "et vos, fratres" (and you, brethren) into "et tibi, pater" and "et te, pater" (and you, Father) when reciting his own Confiteor.
Occasions of recitation
Until 1969, therefore, the Confiteor was spoken (not sung) twice at the beginning of Mass, after the recitation of Psalm 42/43, once by the priest and once by the server(s) or by the deacon and subdeacon. It was also said, once only (not by the priest), before Communion was distributed to the faithful, until Pope John XXIII in his 1960 Code of Rubrics had it omitted when Communion was distributed within Mass. As the Tridentine Missal did not envisage any distribution of Communion to the faithful within Mass, it was the rite of giving Communion to the faithful outside of Mass that was used even within Mass.
The Roman Ritual also required recitation of the Confiteor before administration of Extreme Unction and the imparting of the Apostolic Blessing to a dying person. The Ritual's prescription that a penitent should begin their confession by reciting at least the opening words of the Confiteor was not generally observed.
The Caeremoniale Episcoporum of the time also laid down that, when a bishop sings high Mass, the deacon should sing the Confiteor after the sermon and before the bishop granted an indulgence. This custom, the only occasion on which the Confiteor was to be sung, rather than recited, had fallen into disuse even before the twentieth century.
In the Roman Missal as revised in 1969 the Confiteor is said only once, by priest, ministers and people jointly, at the beginning of Mass. It may be replaced by one or other of two other forms of introductory penitential act.
The other liturgical books mentioned (the Roman Ritual, the Caeremoniale Episcoporum and the Liturgy of the Hours) no longer require recitation of this particular prayer.
Accompanying gestures and prayers
Tridentine editions of the Roman Missal prescribed that the priest should make a profound bow to the altar while reciting the Confiteor with joined hands and that he should remain bowed until the server or servers began their recitation of the Confiteor.
They also prescribed that, at the words "mea culpa, mea culpa, mea maxima culpa", those reciting the Confiteor should strike their breast three times. Editions of the Roman Missal since 1970 do not specify the number of times. This prescribed “striking” is a symbolic tapping of the chest with a clenched fist over one’s heart, signifying remorse. This gesture of sorrow for sin is found in Scripture, as for instance in Luke 18:13 and Jeremiah 31:19.
Tridentine editions prescribed that a prayer be said for the person who recited the Confiteor. After the priest's recitation, the server(s) prayed: "Misereátur tui omnípotens Deus, et dimíssis peccátis tuis, perdúcat te ad vitam ætérnam" (May Almighty God have mercy upon you and, your sins having been forgiven, may He bring you to eternal life). And the priest responded: "Amen." After the recitation by the server(s), the priest said the same prayer (with vestri and vestris, "you" plural, not "you" singular), and the server(s) answers: "Amen." In editions since 1970, in which the Confiteor is recited jointly, this prayer is said by the priest alone, replacing vestri and vestris ("you" and "your") with nostri and nostris ("us" and "our"). The official English translation is: "May almighty God have mercy on us, forgive us our sins, and bring us to everlasting life."
Tridentine editions of the Roman Missal included a second prayer of absolution, said by the priest alone: "Indulgéntiam, absolutiónem, et remissiónem peccatórum nostrórum tríbuat nobis omnípotens et miséricors Dóminus" (May the Almighty and merciful God grant us pardon, absolution, and remission of our sins). The server(s) or deacon and subdeacon responded to this also with "Amen."
Usage in Lutheranism
I confess to God Almighty, before the whole company of heaven, and to you, my brothers and sisters, that I have sinned in thought, word, and deed; in what I have done and in what I have failed to do, by my fault, by my fault, by my most grievous fault; wherefore I pray God Almighty to have mercy on me, forgive me all my sins, and bring me to everlasting life. Amen.
- Fortescue, Adrian. "Confiteor." The Catholic Encyclopedia Vol. 4. New York: Robert Appleton Company, 1908. 29 August 2016
- Griffin, Patrick. "Rites." The Catholic Encyclopedia Vol. 13. New York: Robert Appleton Company, 1912. 29 August 2016
- Wasserschleben, Friedrich Wilhelm (1851). Die Bussordnungen der abendländischen Kirche. Halle: Ch. Graeger. p. 555. Retrieved 24 February 2015.
- The saints before whose relics confession is made
- From the 2010 ICEL Translation
- Missale Romanum 1962
- Prayers at the Foot of the Altar
- Ritus servandus in celebratione Missae, III, 8 (p. LV in the 1962 Missal)
- Code of Rubrics, 503
- MacMichael, Brian W., "The New Translation of the Holy Mass: The Confiteor", Diocese of Fort Wayne-South Bend
- (Lutheran Service Book, Divine Service I)