The Magna Roman Empire was the ruling body of Magna Roma for thousands of years before the 24th century and was ruled by an Emperor.
Following first contact with the USS Enterprise, Christianity had flourished with Emperor's approval. (TOS reference: The Autobiography of James T. Kirk)
The Empire transformed rapidly into the Republic of Magna Roma. The republic was then able to join the United Federation of Planets. (TNG novel: The Captains' Honor)