Epistemology is the investigation of what distinguishes justified belief from opinion, or what we call “knowledge.” Epistemic has to do with knowledge or the degree to which to which we validate it.
When you limit what constitutes truth, even the possibility of truth, or where truth can be found, you have closed off an important part of how we gain knowledge. This is bias.
When you use ways of thinking that don’t follow implications (what is logically implied by honestly evaluating how one thing leads to another or what needs to be true for something to exist [entails]), you have effectively created an echo chamber.
Now imagine the combination. Most often what happens with his system of thought is you rationalize away a possible truth (cognitive dissonance) because it doesn’t fit what you want to believe (bias) by either distorting the implications to fit your bias or denying it outright.
This is epistemic closure. You have built a seal around what you allow yourself to know.