St. Michael is an Angel, so either has existed since before the Earth was made or if you want the earliest literary reference from the Book of Enoch which dates to around 300 BCE but might have existed in an oral tradition before that. Earth being round dates to around 500 BCE so depending on how old the oral tradition of the angel Michael were around was probably earlier but maybe contemporary.
> "Once upon a time, there were four little angels who went to the police academy and they were each assigned very hazardous duties. But I took them away from all that and now they work for me. My name is YHWH." —not Aaron Spelling