There have been many people to claim to either be God, or speak directly on its behalf. There have been many more to testify to miracles that prove one version of spiritual metaphysics or another. The problem is that nearly all the claims are unverifiable, and none of them fall within something like my personal experience with this universe.
So, I am left with sincere doubt. I can't merely just decide to be a particular brand of theist, as I still wouldnt really believe in it. I would be lying to everyone and myself, to say that I actually thought like they did, when, I doubt I have anything like the capacity to do so. I would consider this pretend sort of theist to be a greater insult to your position and beliefs than my current agnosticism.
I have to treat claims equally; it's a bit of a pain to be consistent. I can no more just pick one and fly with it outside of what my personal experience tells me than you could just decide for a week to believe in Zeus (as discussed earlier).
Your claim to a doctrine with special metaphysical knowledge is a bit hollow, because, well, nearly every theist claims that their metaphysical belief system is the best.