In the movie Rashomon (11th century Japan) a medium is used to relay the account of a deceased victim. In an historical fiction book set in the 12th century (A Morbid Taste for Bones by Ellis Peters), a vision comes to one of the priests in a church and on this basis alone the church members journey to acquire a relic. Were the accounts of priests or shamans always accepted unconditionally even by those in authority?
I only know about medieval Europe, hopefully someone else can come answer about Japan. Not all mystical visions were accepted-- sometimes, people were judged to be either lying or making it up and were punished by the church or local authorities. When they were believed, often it started by persuading a few local people of what they had seen, and then word of the local miracle started to spread, and often it was eventually acknowledged by the church.
In viking era scandinavia visions were often taken seriously, especially if predicted by women. A völva predicted ragnarök and it spead from Sweden to Iceland, her vision has had a massive impact. Another example is Herdis in laxerdala saga whose vision causes the grave of a völva to be moved. Usually visions meant something similar, a body had to be reburied or a habit had to be changed.