In Queen of the Darkest Hour, I have a newly baptized Christian swearing an oath on saints’ relics. A lot of saints’ relics. It is possible, if the holy objects are tiny enough for a portable reliquary.
Medieval portable reliquaries were the size and shape of a purse. It is possible a pilgrim could visit multiple sites, collect miniscule, ordinary looking objects, and house them inside the portable reliquary before it was gilded with metal and decorated.
Portable reliquaries allowed pilgrims to be in the presence of a saint even when they had returned home. They were also handy in politics, when someone promised to be a vassal to his lord.
In Queen, this new Christian was an important person—it would be a spoiler for me to identify him—and he was provided with two reliquaries for his vow to King Charles, my heroine’s husband: “By the bone fragments of the holy martyrs Ewald the Fair and Ewald the Black, the hairs of the holy martyr Ursula, the oil of the lamps burning above the tomb of the holy martyr Boniface, the stone chip from the tomb of Saint Lioba, the dust from Saint Willibald’s and Saint Walburga’s graves, and a splinter from the True Cross, I make this oath and will keep it all my days, so help me God, creator of the heavens and earth.”
For more about portable reliquaries, see my post on English Historical Fiction Authors.