OK. Then I'm left wondering why self-declarations about beliefs in Gods and Goddesses are so surely true while other things people say are not so credible. I do see how your making that distinction helps your argument seeing that your word on your not believing in a God is all the evidence you have. It's common in rhetoric to make assertions if confirming evidence is lacking. I've seen Christian apologists make such assertions when they lack evidence for their claims.
Oh, that’s easy and it’s been mentioned already.
Because a BELIEF inherently has little to no evidence. It’s an OPINION. So when someone tells you that is their belief, then their opinion on the matter is likely the best possible evidence available. Add that to this forum where there is nothing to be gained by lying about it.
Your assertion is that you know MORE than the person describing their own beliefs, and that’s just so un-useful as to be laughable.
Many of the people who post here have posted lengthy and thoughtful stories over the years about their interaction with theists, theism and religion. This includes their behaviors, reactions, studied self-analysis and emotions. The evidence of their stories aggregates to a collection of evidence that, through consilience and consistency, makes for a significantly more compelling evidence than some new guy denigrating us all and calling us deluded.
In short; you have established yourself as a poor witness of our beliefs, and there are better witnesses available.