I recently discovered that the Philippine Star has daily theological one-liners printed at the bottom margin of the back page. You can't miss it. It's in large type, with a bright yellow band background to highlight it. Some examples: "Persistence in prayer pleases God" (May 3, 2008), "If you fill your heart with God's Word, He'll bring spiritual health to your soul" (June 5, 2008), "God is always in control behind the scenes" (June 6, 2008). Given these claims, we can rule out the possibility that they're talking about the god of the Deists (who neither gave any commandments nor is interested in the affairs of the universe) or the god of Episcopalian Bishop John Shelby Spong or that of theologian Paul Tillich (a nebulous "force" which he defines as, among other things, the "ground of all being,").
But whoever the deity in question may be, what I'd like to know from those who declare and believe in the above declarations is how they know they are so. What method did they employ to discover these "facts"? How did they arrive at their knowledge of these things? Are they just parroting others? Could it be that they inferred them from various older claims, premises which are not yet known and haven't been evinced to be true? Did they merely find them in some ancient text; if so by what means did the author of that text discover their veracity? How can others--like you and me--determine/confirm the claims? (Can we, for example, have an audience with this entity and get it straight from the horse's mouth? What objective tests can we perform?) Or--Zeus forbid--could it be that those who say the above things merely believe and want/wish them to be true? Until the epistemological questions are answered the claims are unsupported and unsubstantiated. And just to repeat an epistemic heuristic: extraordinary claims demand extraordinary quality of evidence in their favor.
And so it is for all other theological claims. The query "How do you know it's true?" invariably exposes the absence of a valid epistemic grounding for the belief. Thus were we to ask, "How do you know that the phenomena 'God' and 'soul' you speak of in the above claims in fact exist and are real?", we would be met with answers that don't at all substantiate the claims. It goes without saying that it would be imprudent or even foolish to believe in anything lacking sufficient justification/evidence. One can choose to believe or one can prefer to believe rather than withhold belief, but there is no rational imperative to do so. In fact it would either be nonrational or irrational to choose/prefer to believe.
Psychologically, it is intriguing to note that while most people will be skeptical were you to tell them that a new animal species has been discovered, say, a whale with legs and feet, the same people will not blink but instead swallow hook, line, sinker, and even fishing pole when you tell them there exists an invisible, undetectable entity that is over 14 billion years old, one that has an understanding of chemistry, biology, and physics that surpasses a billion Einsteins combined, that has perfect telekinetic, telepathic and clairvoyant abilities, that can speak and understand any language (past, present and future), including all animal talk, and one which wants you to verbally communicate with it regularly but more importantly enjoins you to share with it part of your income every Sunday through human delegates (albeit self-appointed). Thus, while we would expect that the more implausible (the more outrageous) the claim the more skeptical people are, in reality, it doesn't follow. The fact is there are very implausible, extraordinary claims for which there is no good evidence whatsoever that people believe in through and through (quite unthinkingly).
How do you know I'm telling the truth were I to say that I always carry a million Euros--cash--on me wherever I go? Why is it that your gut reaction is to be skeptical? How do you know that you've really been cured by a Touch Therapist after she repeatedly passes her hands 3 inches above your body for several minutes? How do you determine the efficacy of this treatment modality? And even prior to that, given our fund of anatomical/physiological understanding, how do you know whether Touch Therapy is even remotely plausible? How do self-proclaimed "alien abductees" know that the bruises and marks on their bodies were really caused by extraterrestrials and not by some other means? How do you know they've explored and ruled out all other possible explanations? How do you know that goddess Kali is real, alive, and is truly out there somewhere in some dimension? How do you know she isn't? If you don't know she isn't and can't ever prove she isn't, then how do you know that some other deity (perhaps your own) is the real one if the reasons for saying it exists and is the real one are no better, no more persuasive (much less conclusive) than those for Kali?
How do you know it's true? How do you know when you should believe and when you should remain skeptical, how much to believe in claims and how much to question them?
How do you know?