In 569 the Byzantine emperor Justin II sent a relic of the True Cross to Radegund, the Thuringian princess who had left her Frankish royal marriage to found the monastery of the Holy Cross at Poitiers. For its solemn reception on 19 November of that year, the Italian poet Venantius Fortunatus, then living at Poitiers, composed Vexilla regis prodeunt, the banners of the king go forward, and the hymn's first audience watched an actual procession, relic, clergy and royal foundress, as it was sung.
Fortunatus imagined the cross as the vexillum, the Roman military standard, advancing before the King of heaven, and built the hymn on the paradoxes the cross had gathered in Christian imagination, the tree on which life himself died and by death brought forth life, the wood honoured to bear the ransom of the age and to weigh the price of the world.
One stanza quotes and corrects the psalter, God has reigned from the tree, an old Christian reading of Psalm 96. The address to the cross itself, O crux ave, spes unica, hail O cross, our only hope, became one of the most repeated lines in Latin Christianity, engraved on monuments and gravestones wherever the language reached, the whole hymn a case of court poetry, relic politics and theology fused into permanent liturgy.