The Roman Catholic Church and the Eastern Orthodox Church are the churches in which the veneration of saints is most common. They do not, however, honor all the same individuals as saints. Some of the reasons for differences are historical, rooted in the Great Schism. Others are local. In the Early Christian Church, treatment as a saint depended on local and regional recognition of an individual's sanctity and reputation of miracles. Most saints had only local devotional cults, and only the most famous - the apostles, the companions of Christ, persons mentioned in Scripture, and very few international celebrities - developed wide-spread devotion.
The Western church developed its institutionalized system of canonization after the [Great schism]?, so the highly organized calendar of observation of saints' days seen in Roman Catholicism and a few of its break-away churches (the Anglican communion has partially preserved the idea of an organized attention to the saints) is not seen in the Orthodox churches.
Even inside the Roman Catholic church, there are different extents of devotion. Some saints' days are observed only in one diocese? {I don't know of any actual examples of that, but it's canonically possible}. Many are honored as saints in their own home region, and others are honored as saints only by a particular religious order. For instance, each monastic order honors many individuals with special saint-days who are ignored in the wider structure of parishes.
This difference does not mean that the Western and Eastern churches do not admit to the validity of holy individuals in the other parts of the Church, but that they are not interested in each others lists or calendars.
A number of people are venerated as saints who may never have actually existed; the polite term for this is 'ahistorical.' Sorting out exactly which saints are historical is difficult, because of the larger difficulty of proving a negative: the absence of independent records of a saint's existence don't prove she or he never existed, because there are no specific records of the existence of many people who lived before the 20th century.
Related to this, some pre-Christian deities have been adopted as saints. Some cults seem to have been Christianized fairly directly--for example, Saint Brigid was a goddess before Christianity ever reached Ireland. In other cases, older beliefs and legends may have been grafted onto the lives of humans who are venerated as saints.
The converse of this is the idea that not all saints are known to any church. For example, anyone who died for his or her Christian belief is counted as a saint, whether or not anyone knows about the martyrdom. The doctrine is that God creates saints, and the church merely recognizes them: even if no church knows of a martyr, God does.
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