Canonization is the declaration of a deceased person as an officially recognized saint, specifically, the official act of a Christian communion declaring a person worthy of public veneration and entering their name in the canon, or authorized list, of that communion's recognized saints.
Canonization is a papal declaration that the Catholic faithful may venerate a particular deceased member of the church. Popes began making such decrees in the tenth century. Up to that point, the local bishops governed the veneration of holy men and women within their own dioceses; and there may have been, for any particular saint, no formal decree at all. In subsequent centuries, the procedures became increasingly regularized and the Popes began restricting to themselves the right to declare someone a Catholic saint. In contemporary usage, the term is understood to refer to the act by which any Christian church declares that a person who has died is a saint, upon which declaration the person is included in the list of recognized saints, called the "canon".
By the fourth century, however, "confessors"--people who had confessed their faith not by dying but by word and life--began to be venerated publicly. Examples of such people are Saint Hilarion and Saint Ephrem the Syrian in the East, and Saint Martin of Tours and Saint Hilary of Poitiers in the West. Their names were inserted in the diptychs, the lists of saints explicitly venerated in the liturgy, and their tombs were honoured in like manner as those of the martyrs. Since the witness of their lives was not as unequivocal as that of the martyrs, they were venerated publicly only with the approval by the local bishop. This process is often referred to as "local canonization".
This approval was required even for veneration of a reputed martyr. In his history of the Donatist heresy, Saint Optatus recounts that at Carthage a Catholic matron, named Lucilla, incurred the censures of the Church for having kissed the relics of a reputed martyr whose claims to martyrdom had not been juridically proved. And Saint Cyprian (died 258) recommended that the utmost diligence be observed in investigating the claims of those who were said to have died for the faith. All the circumstances accompanying the martyrdom were to be inquired into; the faith of those who suffered, and the motives that animated them were to be rigorously examined, in order to prevent the recognition of undeserving persons. Evidence was sought from the court records of the trials or from people who had been present at the trials.
Augustine of Hippo (died 430) tells of the procedure which was followed in his day for the recognition of a martyr. The bishop of the diocese in which the martyrdom took place set up a canonical process for conducting the inquiry with the utmost severity. The acts of the process were sent either to the metropolitan or primate, who carefully examined the cause, and, after consultation with the suffragan bishops, declared whether the deceased was worthy of the name of 'martyr' and public veneration.
Though not "canonizations" in the narrow sense, acts of formal recognition, such as the erection of an altar over the saint's tomb or transferring the saint's relics to a church, were preceded by formal inquiries into the sanctity of the person's life and the miracles attributed to that person's intercession.
Such acts of recognition of a saint were authoritative, in the strict sense, only for the diocese or ecclesiastical province for which they were issued, but with the spread of the fame of a saint, were often accepted elsewhere also.
In the Catholic Church, both Latin and constituent Eastern churches, the act of canonization is reserved to the Apostolic See and occurs at the conclusion of a long process requiring extensive proof that the candidate for canonization lived and died in such an exemplary and holy way that they are worthy to be recognized as a saint. The Church's official recognition of sanctity implies that the person is now in Heaven and that they may be publicly invoked and mentioned officially in the liturgy of the Church, including in the Litany of the Saints.
For several centuries the bishops, or in some places only the primates and patriarchs, could grant martyrs and confessors public ecclesiastical honor; such honor, however, was always decreed only for the local territory of which the grantors had jurisdiction. Only acceptance of the cultus by the Pope made the cultus universal, because he alone can rule the universal Catholic Church. Abuses, however, crept into this discipline, due as well to indiscretions of popular fervor as to the negligence of some bishops in inquiring into the lives of those whom they permitted to be honoured as saints.
In the Medieval West, the Apostolic See was asked to intervene in the question of canonizations so as to ensure more authoritative decisions. The canonization of Saint Udalric, Bishop of Augsburg by Pope John XV in 993 was the first undoubted example of papal canonization of a saint from outside of Rome being declared worthy of liturgical veneration for the entire church. 
Thereafter, recourse to the judgment of the Pope occurred more frequently. Toward the end of the 11th century the Popes began asserting their exclusive right to authorize the veneration of a saint against the older rights of bishops to do so for their dioceses and regions. Popes therefore decreed that the virtues and miracles of persons proposed for public veneration should be examined in councils, more specifically in general councils. Pope Urban II, Pope Calixtus II, and Pope Eugene III conformed to this discipline.
Hugh de Boves, Archbishop of Rouen, canonized Walter of Pontoise, or St. Gaultier, in 1153, the final saint in Western Europe to be canonized by an authority other than the Pope: "The last case of canonization by a metropolitan is said to have been that of St. Gaultier, or Gaucher, [A]bbot of Pontoise, by the Archbishop of Rouen. A decree of Pope Alexander III [in] 1170 gave the prerogative to the [P]ope thenceforth, so far as the Western Church was concerned." In a decretal of 1173, Pope Alexander III reprimanded some bishops for permitting veneration of a man who was merely killed while intoxicated, prohibited veneration of the man, and most significantly decreed that "you shall not therefore presume to honor him in the future; for, even if miracles were worked through him, it is not lawful for you to venerate him as a saint without the authority of the Catholic Church." Theologians disagree as to the full import of the decretal of Pope Alexander III: either a new law was instituted, in which case the Pope then for the first time reserved the right of beatification to himself, or an existing law was confirmed.
However, the procedure initiated by the decretal of Pope Alexander III was confirmed by a bull of Pope Innocent III issued on the occasion of the canonization of Cunigunde of Luxembourg in 1200. The bull of Pope Innocent III resulted in increasingly elaborate inquiries to the Apostolic See concerning canonizations. Because the decretal of Pope Alexander III did not end all controversy and some bishops did not obey it in so far as it regarded beatification, the right of which they had certainly possessed hitherto, Pope Urban VIII issued the Apostolic letter Caelestis Hierusalem cives of 5 July 1634 that exclusively reserved to the Apostolic See both its immemorial right of canonization and that of beatification. He further regulated both of these acts by issuing his Decreta servanda in beatificatione et canonizatione Sanctorum on 12 March 1642.
In his De Servorum Dei beatificatione et de Beatorum canonizatione of five volumes the eminent canonist Prospero Lambertini (1675-1758), who later became Pope Benedict XIV, elaborated on the procedural norms of Pope Urban VIII's Apostolic letter Caelestis Hierusalem cives of 1634 and Decreta servanda in beatificatione et canonizatione Sanctorum of 1642, and on the conventional practice of the time. His work published from 1734 to 1738 governed the proceedings until 1917. The article "Beatification and canonization process in 1914" describes the procedures followed until the promulgation of the Codex of 1917. The substance of De Servorum Dei beatif?catione et de Beatorum canonizatione was incorporated into the Codex Iuris Canonici (Code of Canon Law) of 1917, which governed until the promulgation of the revised Codex Iuris Canonici in 1983 by Pope John Paul II. Prior to promulgation of the revised Codex in 1983, Pope St. Paul VI initiated a simplification of the procedures.
The Apostolic constitution Divinus Perfectionis Magister of Pope John Paul II of 25 January 1983 and the norms issued by the Congregation for the Causes of Saints on 7 February 1983 to implement the constitution in dioceses, continued the simplification of the process initiated by Pope Paul VI. Contrary to popular belief, the reforms did not eliminate the office of the Promoter of the Faith (Latin: Promotor Fidei), popularly known as the Devil's advocate, whose office is to question the material presented in favor of canonization. The reforms were intended to reduce the adversarial nature of the process. In November 2012 Pope Benedict XVI appointed Monsignor Carmello Pellegrino as Promoter of the Faith.
Candidates for canonization undergo the following process:
Canonization is a statement of the Church that the person certainly enjoys the Beatific Vision of Heaven. The title of "Saint" (Latin: Sanctus or Sancta) is then proper, reflecting that the saint is a refulgence of the holiness (sanctitas) of God himself, which alone comes from God's gift. The saint is assigned a feast day which may be celebrated anywhere in the universal Church, although it is not necessarily added to the General Roman Calendar or local calendars as an "obligatory" feast; parish churches may be erected in their honor; and the faithful may freely celebrate and honor the saint.
Although recognition of sainthood by the Pope does not directly concern a fact of Divine revelation, nonetheless it must be "definitively held" by the faithful as infallible pursuant to, at the least, the Universal Magisterium of the Church, because it is a truth related to revelation by historical necessity.
Popes have several times permitted to the universal Church, without executing the ordinary judicial process of canonization described above, the veneration as a saint, the "cultus" of one long venerated as such locally. This act of a Pope is denominated "equipollent" or "equivalent canonization" and "confirmation of cultus". In such cases, there is no need to have a miracle attributed to the saint to allow their canonization. According to the rules Pope Benedict XIV (regnat 17 August 1740 - 3 May 1758) instituted, there are three conditions for an equipollent canonization: (1) existence of an ancient cultus of the person, (2) a general and constant attestation to the virtues or martyrdom of the person by credible historians, and (3) uninterrupted fame of the person as a worker of miracles.
As examples, prior to his pontificate, of this mode of canonization, Pope Benedict XIV himself enumerated the equipollent canonizations of saints:
Later equipollent canonizations include those of saints:
Pope Francis added saints:
The majority of Protestant denominations do not formally recognize saints, believing that all Christian believers (living and dead, and regardless of the specific branch of Christianity to which they adhered) are considered saints. However, some denominations do, as shown below.
The following terms are used for canonization by the autocephalous national Eastern Orthodox Churches? or  "glorification" (Russian Orthodox Church), kanonizats'ia (Georgian Orthodox Church), (Serbian Orthodox Church), canonizare (Romanian Orthodox Church), and (Bulgarian Orthodox Church). The following terms are used for canonization by other autocephalous Eastern Orthodox Churches:  (Katharevousa) agiokatataxi/agiokatataxis, "ranking among saints" (Ecumenical Patriarchate of Constantinople, Church of Cyprus, Church of Greece), kanonizim (Albanian Orthodox Church), kanonizacja (Polish Orthodox Church), and kanonizace/kanonizácia (Czech and Slovak Orthodox Church).
The Orthodox Church in America, an Eastern Orthodox Church partly recognized as autocephalous, uses the term "glorification" for granting official recognition to someone as a saint (see glorification).
Within the Armenian Apostolic Church, part of Oriental Orthodoxy, there had been discussions since the 1980s about canonizing the victims of the Armenian genocide. On 23 April 2015, all of the victims of the genocide were canonized.
Those proposed as coming under the definition of cases excepted (casus excepti) by Urban VIII are treated in another way. In such cases it must be proved that an immemorial public veneration (at least for 100 years before the promulgation, in 1640, of the decrees of Urban VIII) has been paid the servant of God, whether confessor or martyr. Such cause is proposed under the title of "confirmation of veneration" (de confirmatione cultus); it is dealt with in an ordinary meeting of the Congregation of Rites.
In 1660 the convocations of Canterbury and York canonized King Charles.