San Marino and the romans?

I have asked this before and only received answers stating that the roman empire was gone by the time of San Marino's foundation. However the foundation story of Marino states it was founded in the year 301 a.d., well within the years of the empire for those who will attempt to answer this question so no answers saying my question is invalid from someone who thinks they know and get basic dates wrong.

So the question is this, how did Christians manage to form an enclave in the roman heartland completely autonomous from Rome during the Christian persecutions. So again without saying I'm wrong because the Romans wernt around, is the foundation story a myth or if it's true how was San Marino able to maintain its independence from Rome? Geographic defenses can't be the reason as roman armies were able to think of ways to overcome any natural defense (I'm thinking of Masada). So is the foundation story a myth or based in fact and how did they maintain independence from the romans if the story is true.

P.S. The Roman empire, more specifically he western empire, lasted well into the 5th century so poser historians stop saying Rome wasn't around or you just make a fool of yourself. Rome was still in full command of itself at the time of Marino's supposed founding, so if you answer that your dead wrong. So someone who knows help me out.