Final answer:
The country where Marinus of Arba founded a monastic community after fleeing a decree by Diocletian is San Marino, known for its early monasticism and enduring religious institutions.
Step-by-step explanation:
The country that was home to a monastery founded by Marinus of Arba, who left his hometown after Emperor Diocletian issued a decree to rebuild city walls in Rimini, is San Marino. According to tradition, Marinus, a Christian stonemason, founded a monastic community on Monte Titano in 301 CE following the decree of Diocletian which led him to seek a place where he could practice his faith without persecution.
This led to the establishment of the Republic of San Marino, one of the world's oldest republics, which is known for its historical monasticism and religious institutions.