Marinus (of the sea) was born in c. 275 in what is now Croatia, and later, along with his friend the future Saint Leo emigrated to Rimini (aka Italy), where they found work as a stonemasons and became Deacons of the Catholic church. Eventually the two would part ways, with St. Leo heading to Monte Feretro, now known as San Leo, and Marinus heading off for the hills of Monte Titano, where he intended to build a chapel and monastery and live as a hermit.
But the hermit attracted a crowd with his teachings of Christ. One day, the owner of the mountain, Felicissima, arrived from Italy and she was so knocked out by his teachings that she converted and gifted him the mountain, and on September 3, in the year of our Lord three-hundred and one, the state of San Marino was founded.
Legend has it (all the facts around this guy are a wee hazy, to be honest) that before his death in 366, the last words he ever spoke were "Relinquo vos liberos ab utroque homine" or "I leave you free from both men," with the "men" in question being the Emperor and the Pope, which, if you were to be liberated from any two men, especially, in 366, it'd have to start with those two, right?
At 24 square miles, San Marino ranks just 191st among the world's countries, and its population of 33,860 ranks 217th according to the UN, but San Marino ranks #1 in the world as oldest nation state and constitutional republic, and that ain't nothing.