If you believe that all crows are black without ever having any experience with crows or birds, then that belief does not qualify as knowledge. It may be true - but you weren't rationally justified in believing it.
Similarly, if you believe that all dogs are large because you have never encountered a small dog, that also would not qualify as knowledge. The belief may have some reasonable justification, but the claim simply isn't true. Verification is the process by which we make sure that what hear has some basis in truth. This is vital because we are fallible - it isn't true that any idea which occurs to us is correct. Since it makes no sense to say that a person knows something which is false (i.e., that 2 + 2 = 5), we must make some effort to verify information before being able to properly say that we know it.
So when someone says they know a god exists, what are they actually saying? Aren't they saying they hope a god exists, that they feel that a god exists.. that it feels right to them to hold such a position? Is it actually knowledge, or even belief?