Certain things are a matter of belief. 
Do you believe, for example, that the holy grail is real and that its location has or has not yet been discovered, or do you believe it is a striking creation of medieval European literature? If it is not actually real, why do so many claim to have found it? Why does something of such ambiguity hold such sway, spanning centuries and continents? 

Belief in certain things is a matter of contention. Is there life after death? God? Reincarnation? Telekinesis? Whereas belief in other things is rarely of dispute. Rain falls to the ground from clouds. The sun rises in the east and sets in the west. Kyoto city was the capital of Japan for over 1,000 years. 

So why is the existence of a god or gods or goddesses or holy people or deities or kami for that matter, at times hotly contested whereas the sun’s emergence in the east generally is not?

We might start by pointing to the tangible-to-the five-senses and thusly physically experiential nature of the sunrise in what we know to be “east”. However it is also true is it not, that many people, when asked where east is, will not be able to answer correctly on the spot without external reference to a map or mechanical compass of some sort. Physical experience alone is insufficient. To understand where east is, we must have somehow learned something about the relationship between the sun and earth, about planetary orbits, the solar system and space. But from the point of view of Pluto, or of a star in Andromeda, how fixed or relevant ultimately is Earth’s east, anyway? Is east to the left or to the right? Your left or my right? 

As for intangible-to-the-five-senses things such as kami, we might point to the ubiquity and continuity of shrines in Japan as evidence of their actual existence. “Believers” may claim to have experienced physically tangible results after praying to kami, such as healed illnesses or passed university entrance examinations. Visions of non-physical beings or other experiences beyond the tangible-to-the-five-senses might also be cited by some as proof that kami are real. Such “intangibles” however, are often disputed as imagination and thusly unacceptable and unverifiable evidence. But what are “visions” after all, if not something seen somehow?    

I am not here to profess what is true or not true. But I am here to question our assumptions about what we believe to be true or not true. And I am here to suggest that the holy grail itself is not necessarily its purported historical and physical reality, and that it is not necessarily a thing of literary legend—but that perhaps the real holy grail is realizing that what we believe to be true is “true”. Truth is a consequence of belief; not necessarily the other way around. What you believe creates what you experience; not necessarily the other way around.
Perhaps, in other words, the real holy grail is that we are all—each and every one of us—kami.