Here’s an excerpt from Edward Feser’s Scholastic Metaphysics: A Contemporary Introduction (Page 30):
…metaphysics is prior to epistemology. One way in which this is the case is that absolutely every epistemological theory rests on metaphysical assumptions—including Hume’s when he begins with the supposition that there are impressions and ideas, and including the naturalist’s when he supposes that our cognitive faculties are at least reliable enough to make natural science an objective enterprise. Naturally, these metaphysical assumptions cannot be justified by reference to the epistemological claims they support without begging the question. When the critic of metaphysics insists that the metaphysician establish his epistemological credentials before making any metaphysical assertions, he is making a demand that is incoherent and to which he does not submit himself.
Another way in which metaphysics is prior to epistemology is that our knowledge of various metaphysical truths is something with which a sound epistemology must be consistent, so that if an epistemological theory is not consistent with our having knowledge of these truths then it must be rejected. In the limiting case, an epistemological theory that was inconsistent with its own metaphysical assumptions would obviously be for that reason something we must reject.Feser’s notes that there are several metaphysical truths that cannot be coherently denied, and if naturalism and scientism are not facilitating our knowing of such truths, then we can regard naturalism and scientism as false doctrines.