Problems with the Standard Definition of Knowledge

While I think that the so-called standard definition of truth (justifiable true belief) the best description of knowledge, it does have some serious problems. First, truth is presupposed in the defintion. If knowledge is supposed to be the way in which we determine if something is true and we have truth already in our defintion of knowledge, then how do we know that what we know is indeed true. We need truth in order to have knowledge, but we define knowledge as something that is already true.

Another point of confusion for me is the belief part of the definition. In the past, science (which is supposed to be our unimpeachable bastion of knowledge, not so?) has touted as true certian theories. Each of these universally-accepted theories were fully justified and believed to be true. Indeed, they were concidered knowledge. But, as time passes, we discover new justifications for new beliefs and they become knowledge. Now, certainly, we now know that the earth is not flat. It was never true that earth was flat, yet, in the past, scientisists possessed justified beliefs that the world was flat AND claimed that that fact was indeed knowledge.

How does the disruption of the Standard Defenition of Knowledge effect the way in which we "know" God exists? I think, for some people, the Standard Defenition will only help to explain why they think God exists. For me, though, it slightly clouds the issue.