In the 1st C. AD, Christian converts began to invade Florentia. The Christians were persecuted—thrown to lions in the amphitheater—on and off throughout the third century A.D. But by 313 a bishop was living safely in Florence; it’s likely that the first Christian cathedral was built around this time too.
The Church of San Salvador was the name of the edifice, and its location was quite near if not directly under what is now the Cathedral of Santa Maria del Fiore in the Piazza del Duomo.
Sandwiched between the first cathedral and the last was Santa Reparata, a church named for a twelve-year-old female martyr. The young saint was said to have appeared in the middle of a 5th-century battle between a host of Vandals and Goths and the citizens of Fiesole.
Santa Reparata suddenly arrived on the spot with a bloodred banner and a lily in her hand. Miraculously, following close behind her was the Roman general Stilicho with a fresh legion of troops. The barbarians fought a losing battle, and the Florentines built a new cathedral in remembrance of the girl’s military assistance.
You can read more about the church of Santa Reparata, which would rebuilt as il Duomo later on here: https://operaduomo.firenze.it/blog/posts/la-nuova-musealizzazione-di-santa-reparata
Holler, Anne. Florencewalks: Four Intimate Walking Tours of Florence’s Most Historic and Enchanting Neighborhoods (Kindle Locations 57-65). Henry Holt and Co.. Kindle Edition.