Your definition of "credible" was "able to be believed"
I gave some counter examples of people believing things that are not credible. This falsifies your definition. You need to, at least, qualify who is able to believe a credible thing.
And I asked you for your alternative and you don't have one. I suggest that credible is the wrong word to use since it depends on ones weltbild and therefore the only commonality is it's believability.
In other words it is credible to them. This is why argument from incredulity is a fallacy.