Don't know why people keep bringing this up.Japanese writers seem to treat sainthood like an RPG class similar to priest/healer/white mage.
How is this different from real life. The Catholic church and other christian denominations may bestow the title only posthumously, but it's an acknowledgment that the person was a saint in life, only undiscovered at the time.
This avoids the messy problems of worship of a living person.
For a fantasy world with convenient settings, there's not much reason a saint can't be recognized beforehand.
If this is about saints having powers....saints are supposed to have "powers". To be recognized as a Saint by most denominations, particularly Catholicism, the person needs to have performed 1~2 "verifiable" miracles on behalf of God (aka something supernatural) or sometimes, widespread historical record of being a prolific miracle worker is also allowed.
Saints are one of the only RPG classes with literal basis irl for their fantasy class powers. I don't think there is an official swordsman school of the "Brave Sword Slash" technique, or the Association for preservation of the Necromancer Arts.