First it should be noted that according to the testimony of Sacred Scripture every Christian is a saint. The Greek New Testament speaks in many places of the hagios (Acts 9:32; Rom 15:25, 31; Eph 1:1; Col. 1:2; Jude 1:3 and others). The Latin Vulgate speaks of the sancti, which is rendered in some English translations as the saints and in others as the holy ones. As St. Peter tells Christians, "you are a chosen race, a royal priesthood, a holy nation, a people of his own, so that you may announce the praises of him who called you out of darkness into his wonderful light." The saints are set apart by God through baptism, filled with His divine life (the Kingdom of God within), and called to announce that Kingdom's presence in the world to the whole human race. Thus it is that in the Scriptural usage all of those baptized into Christ and in the state of grace can rightly be called saints.
In another sense, stricter and more technical, the saints are those in whom Christ's victory over sin, the devil and death has not just begun, as it has in us, but has been completed. This is the case when the wayfaring state of earthy life is concluded and the holiness of life attained in the pilgrim's state is realized perfectly in heaven. Even while saying that no one is truly good but God (Mt 19:17), Christ called us to the perfection of goodness, of holiness, "be perfect as the heavenly Father is perfect" (Mt 5:48, Mt 19:21; Col. 4:12, James 1:4), since nothing imperfect will enter into heaven (Rev 21:27).
The early Church understood that only the Christian who followed Christ perfectly would go immediately into the heavenly Jerusalem. Others would enter the purifying fires of purgation "to be made perfect," from which they would not depart until they had "paid the last penny" (Mt 5:26, 1 Cor 3:13, 15). Since perfection was conformity to Christ in His death, a process begun at baptism, the martyr (literally, witness) for Christ was seen to have achieved the goal. Thus, during the age of persecution (from Pentecost to 311 AD) esteem for those Christians who had been killed in hatred of the faith (in odium fidei) lead them to extol their example of heroic witness to Christ, to guard and preserve their relics (the trophies of victory over death) and to celebrate the anniversary of their birthday into eternal life. The Circular Letter of the Church of Smyrna on the Martyrdom of St. Polycarp (155 AD) illustrates this esteem perfectly.
We have at last gathered his bones, which are dearer to us than priceless gems and purer than gold, and laid them to rest where it was befitting they should lie. And if it be possible for us to assemble again, may God grant us to celebrate the birthday of his martyrdom with gladness, thus to recall the memory of those who fought in the glorious combat, and to teach and strengthen by his example, those who shall come after us.
Finally, the greatest tribute of honor that could be rendered to the martyr was to have his or her name mentioned in the Canon (or Eucharistic Prayer) of the Mass, accompanying the Lord in His Redemptive Sacrifice. This was done on their feast day, the day of their entry into eternal life. The Roman Canon (Eucharistic Prayer 1) retains the eloquent testimony of the Roman Church for the Mother of the Lord, for the apostles, and the most significant martyrs of Rome and Italy.
"In union with the whole Church ...we honor Mary ... Peter and Paul, Andrew, James, John, Thomas, Philip, Bartholomew, Matthew, Simon and Jude; we honor Linus, Cletus, Clement, Sixtus, Cornelius, Cyprian, Lawrence, Chrysogonus, John and Paul, Cosmas and Damian." (Communicantes)
"For ourselves, too, we ask some share in the fellowship of your apostles and martyrs, with John the Baptist, Stephen, Matthias, Barnabas, Ignatius, Alexander, Marcellinus, Peter, Felicity, Perpetua, Agatha, Lucy, Agnes, Cecilia, Anastasia and all the saints." (Nobis quoque peccatoribus)
Thus, in the early centuries of the Church the popular acclaim of sanctity in the martyrs, the veneration of their relics, the honoring of their names in private and liturgical prayer (with the consent of the local bishop) canonized important witnesses to Christ in the universal, and the local, Church, as examples of the perfect fidelity to which all Christians are called.
Although the age of martyrs has never truly ended, the relative peace that existed after the Edict of Milan in 311 meant that martyrdom was a rarer example of perfection than it had been. The Church began to look for other models of holiness, other ways in which conformity to Christ could be a witness to the faithful and the world, the living out in daily Christian life of the dying to self and living for Christ undertaken in baptism. This witness was found in those whose white martyrdom of heroic virtue confessed to the world the triumph of light over darkness, of grace over sin, of the new man over the old man (Eph 4:17-24), and thus of Christ over Satan. Thus, such Confessors, the witness of whose life had the fame of holiness, began to enter the roles of the canonized.
This cultus* (religious veneration) was generally of a single diocese, but as the fame of the person spread it could encompass several dioceses, and in the case of Mary, the apostles and other significant figures be universal in fact. Although the records of early Church Councils shows occasional interventions to correct abuses in the naming of saints and to establish criteria for their acclamation, the process continued to be a local one with some few examples of Popes declaring saints of universal veneration.
The first canonical process seems to be that of Pope Urban II (1089-99), in the "Cause" of Nicholas of Trani. The Bishop of Trani was ordered to conduct a local investigation into his alleged sanctity and miracles, which then would be submitted to the Pope for judgement. This first "Cause" dragged on over several pontificates, and seems not to have been concluded favorably. It also seems to have occasioned developments in the legal procedures themselves, Callistus II (1119-24) requiring all causes to include a critical biography of the Servant of God. As often happens in the Church, abuses brought about major developments in Church practice. In 1170 Pope Alexander III decreed that no one could be declared a saint without the permission of the Supreme Pontiff. This was precipitated by the acclamation as saint of a Swedish "martyr" who was killed while drunk, and thus could not be truly said to be a willing witness for Christ. This regulation was formally incorporated into Church law by Pope Gregory IX in 1234.
The centralization of the canonization process in Rome was an inevitable development of the Church's theological and canonical Tradition. While the acclamation of the faithful and the acceptance of the bishop is in most cases an adequate witness to the holiness of the person, it only provides a moral certainty, a reasonable credibility, that the person is in heaven. In order to give universal witness to the sanctity of someone a higher standard needed to be invoked, that of the charism of the infallibility of the Church. According to Catholic teaching the Church, the Mystical Christ, cannot err in matters of faith and morals (Jn 16:13). The practical exercise of this infallibility falls to the apostolic office, which in the name and by the authority of Christ the Head of the Church intends to bind the faithful in a matter of faith or morals. This can be done either by the college of bishops as a whole, as in a Council (Acts 15:28 15:28), or by the Successor of St. Peter (Lk 22:32, Acts 15:7-12 15:7-12). By the grace of the Holy Spirit Christ protects such judgements of universal import for the Church from error. The common opinion of theologians historically, therefore, is that papal Canonization is an exercise of the charism of infallibility, protecting the Church from raising an unfitting individual to the universal veneration of the faithful. As in the case of a dogmatic declaration, the declaration of a saint inserts that person into the heart of the Church's life, in this case into the central mystery of the faith, the Eucharist, and must by its nature be free from error.
*Cultus. A certain negativity has attached itself to the English term cult (a false, exaggerated religious system) which should not be applied to the older, properly understood, Latin term cultus. The Latin term in the ancient world had the meaning of religious worship of God or a god. It could be applied to the True God (which would be legitimate) or to a pagan god among gods (which would be idolatry). In using the term, but with specific theological meaning, the Church distinguishes between the forms of worship appropriate to God, Trinity, Christ and the Blessed Sacrament (called latria, worship or adoration, in the strict sense), and the forms of veneration and honor appropriate to the Blessed Virgin, the angels and the saints (called hyperdulia or the greatest measure of veneration in the case of Mary and dulia or simple veneration in the case of the angels and other saints). It is a principle of justice that we must honor, respect and show gratitude in proper measure to those who are part of God's plan for our natural and supernatural life. God commands it in the Fourth Commandment. This includes our natural parents who gave us life, but also those to whom we owe a debt for their role in the redemption (1 Cor 4:14-16, Heb. 13:7), first among whom is the Blessed Virgin Mary (Lk 1:48).But without the fidelity of the angels, who served as God's messengers, of the prophets, of the apostles, the evangelists, the Fathers and the great and holy men and women of all ages, we today would not have the faith. That is the foundation of our individual and collective gratitude for the working of God's grace in their lives and thus of their cultus (in the way understood by the Church).