This is probably where the finer points of the argument start to go beyond my present knowledge, but I can tell you how things look from my perspective.

If I have the choice between two churches, one of which claims to have privileged and certain access to the Truth, and another which claims to have access to the Truth, but without the same degree of certainty, I'm going to go with the former, so long as it can provide sufficient evidence to support that claim. The Roman Church's claim to magisterial authority, per se, is not in itself why I remain Catholic (I am a cradle Catholic), but the weight of evidence suggesting it is the church Christ founded makes me trust its claim to authority.

More broadly, I am convicted that there is an absolute truth out there to be known, even if it can be difficult to discover. I know some schools of thought doubt the possibility of epistemic certainty in and of itself, but I've never found those ultimately convincing.

So I hold that there's a truth to be known and that there is evidence pointing to the Catholic Church as the keeper of that truth, and so I trust its Tradition and Magisterium.

Does that get at what you're thinking of regarding epistemic certainty as a premise? Do you think it is not a justified premise?