The first text about an abbey is the 9th-century Latin text Revelatio ecclesiae sancti Michaelis in monte Tumba written by a chanoine living at Mont Saint Michel or at the Cathédrale Saint-André d'Avranches. This text was written at a time of power struggle between Brittany and the County of Normandy against Francia as well as during canon law reforms by Roman emperors.
When Christianity expanded to the area, around the 4th century, Mont Tombe, the original name of Mont Saint Michel, was part of the Diocese of Avranches. By the middle of the 6th century, Christianity had a stronger presence in the bay. By this time, Mont Tombe was populated by religious devotees, hermits (probably some Celtic monks) supplied by the curé of Astériac, who took care of the site and led a contemplative life around some oratories. The hermits Saint Pair and Saint Seubilion dedicated one of the oratories to Saint Étienne, mid-way through the mont and one to Saint Symphorien, at the foot of the rock.
In 710, Mont Tombe was renamed Mont Saint Michel au péril de la Mer ("Mount Saint Michael at the peril of the sea) after erecting an oratory to Saint Michael by bishop Saint Aubert of Avranches in 708. According to the legend, Aubert received, during his sleep, three times the order from Saint Michael to erect an oratory on the Mont Tombe. The archangel was reputed to have left his finger mark on Aubert's skull. This skull is displayed at the Saint-Gervais d'Avranches basilica with such a scar on it.
This sanctuary should be, according to the archangel, a replica of the Gargano in Italy (from the 5th century). Aubert had a local religious artifact removed and instead a circular sanctuary built, made of dry stones. Around 708, Aubert sent two monks to get some artifacts from the Italian sanctuary Gargano (a rock with his foot print and a piece of tissue from the altar). During this mission, the March 709 tsunami is supposed to have destroyed the Scissy forest and turned the Mont into an island. On October 16 709, the bishop dedicated the church and put twelve chanoine there. The Mont-Saint-Michel was born.
The remains of the oratory were found in the chapel Notre-Dame-Sous-Terre. This sanctuary contained the tomb of Aubert and most likely the artifacts brought from Gargano. The chapel Notre-Dame-Sous-Terre is today under the nave of the abbey-church.
The first buildings became too small and under the Western Roman Empire multiple buildings were added. Charlemagne chose saint Michel as a protector of his empire during the 9th century and tried to have the place renamed Mont-Saint-Michel, but during the Middle Ages it was usually called Saint-Michel-aux-Deux-Tombes (Saint Michel with two tombs) (Mons Sancti Michaelis in periculo maris, in reference to Tombelaine).