The poet Venantius Fortunatus wrote it in honor of the arrival of a large relic of the True Cross which had been sent to Queen Radegunda by the Byzantine Emperor Justin II and his Empress Sophia.
Queen Radegunda had retired to a convent she had built near Poitiers and was seeking out relics for the church there.
To help celebrate the arrival of the relic, the Queen asked Fortunatus to write a hymn for the procession of the relic to the church.
The hymn was first sung in the procession (November 19, 569) when the relic of the True Cross was carried in great pomp from Tours to the Queen's monastery of Saint-Croix at Poitiers.
fulget Crucis mysterium,
quo carne carnis conditor
suspensus est patibulo.
Abroad the regal banners fly,
now shines the Cross's mystery:
upon it Life did death endure,
and yet by death did life procure.