My first and foremost faith is in plurality and the truth of subjective reality(the latter of which is based on my study of languages). My polytheistic beliefs are just my natural tendency; I was more or less a polytheist as a child (and I wasn't raised in any sort of theistic household, remember), and so my faith in many Gods (though quite agnostic) is partially based on that. My animistic faith is based on my experiences when meditating, particularly the few times I've done so in nature.
But my faith in the Gods (or rather, Wights in general, not all of which are Gods) is also from my experiences. (Which count for me, but I understand don't count for humanity's collective knowledge, hence I don't assert their positive existence or try to find scientifically verifiable evidence for them; just not a concern of mine).
No one thing I can recall triggered my faith. It is what it is.
It should be noted that a major aspect of my theistic belief is quite archaic. I don't believe in Gods as
explanations, but as
interpretations.
The Modern English word Thunder derives from the Old English word Thunor. The word meant the same phenomenon as today, but was simultaneously the name of its associated God (whom we now call by his Anglicized Old Norse name, Thor). That is to say, Thor is
not, in fact, the "God of Thunder". Thunder
is Thunder. (And he hasn't been around here, lately.
)