On a discursive level, ontology and epistemology are to be understood as equi-primordial and mutually dependent. That is, just as all epistemology utterances implicitly contain ontological presuppostions, so also, all ontological utterances implicitly contain epistemological presuppositions. When it comes to the the ontology of rationality, however, ontology is antecedent to rationality. That is, it is what remains the case about the world regardless of what we believe about it that determines what constitutes what counts as "rational."
For example, according to non-Christianity rationality, the theology of the Trinity makes no sense. On a discursive level, to make the assertion that the doctrine of the Trinity makes no sense, if understood as an ontological statement, implicitly presupposes a non-Christian theory of rationality. To explicitly assert that the epistemological standard of a worldview is legitimate likewise presupposes the legitimacy of a non-Christian ontology. Indeed, such an understanding of the mutual equi-primordiality and dependence of ontology and epistemology, I believe, is the essence of Van Tillian presuppositional apologetics.
From the perspective of the ontology of rationality, however, epistemology and ontology are not mutually equi-primordial. Instead, ontology is antecedent to epistemology and rationality. That is, it is precisely what is actually the case about the world regardless of what we believe about it which determines which standard of rationality is actually rational.
Thus, when someone comes to us and says "the doctrine of the Trinity makes no sense", for us to reply "unless such a God simply does exist"(an ontological utterance) or "unless the Bible really is the infallible word of such a God"(an epistemological statement), from the perspective of discourse, the ontological statements of both parties presupposes epistemological standards and the epistemological statements of both parties presuppose ontological standards. However, when it comes to the priority of the one to the other(epistemology to ontology or vice versa) itself considered on an ontological level, it is ontology which is antedent to epistemology and rationality insofar as what is actually the case determines what sort of manner of thinking makes sense.