The oldest version of the Acts to mention St Genesius, native of Arles, relates that he joined the Roman army when very young, where he performed the role of notary and stenographer. Outraged by the persecution of Christians, he abandoned his post and fled, hiding from his own persecutors. Being a catechumen, he asked to be baptised but the Bishop refused on the grounds of his young age and the unfavourable religious climate. Continuing to flee, he was surprised by his persecutors near the Rhône; he managed to cross to the other bank, but once there he was captured and killed. The Christians took care to preserve his memory in this place, leaving the bloody signs of the crime while carrying his remains to the other bank. This was around the year 303AD, during the persecutions under the Emperors Massiminian and Diocletian.
Leaving aside the controversy surrounding the author of the Passion, what interests us is the testimony of his hagiographer, who claims to have transcribed an oral tradition faithfully into writing. With this authenticity in mind, we can add that the narrative, light on biographical notes, seems to have transmitted the tradition of Genesius to the fifth century almost unchanged in its essentials; but two other testimonies joined this skimpy tradition of the cult of the martyr from Arles, one by Prudentius and one by Venantius Fortunatus. Even though they only make brief mention of him, these two passages, taken in their respective contexts, present Genesius as the martyr of Arles, whose tomb was visited by the Bishop Apollinare of Valenza; we have, moreover, reports from St Gregory of Tours that numerous miracles took place there. The spread of his cult across other cities in Gaul and other regions of Europe produced local versions of the martyr; in the town of San Miniato his cult dates back to at least the seventh century, as is proved by a document from the Church of Arezzo which relates to a Council of Tuscan Bishops that was held in 715 at the parish church of San Genesio in Vico Wallari, on the Via Francigena. When San Miniato destroyed Vico Wallari in the first thirty years of the thirteenth century, the name of that ancient parish church was transferred to San Miniato's Chiesa di Santa Maria, which went on to become a cathedral in 1622. Pope Gregory XV made Genesius patron saint of the diocese on account of the ancient and deep-rooted cult tradition that had always venerated him in this area. Pontifical High Mass is celebrated on his feast day by the Bishop and all the priests of the diocese. In the past a procession would also make its way through the town streets.
Sources: AA.VV. Enciclopedia dei Santi, Bibliotheca Sanctorum, Città Nuova Editrice, Roma, 2013 sanminiato.chiesacattolica.it