The
Roman emperor Constantine I created a new military standard for his army which displayed the first two
Greek letters of the name of Christ -
Chi (χ) and
Rho, (ρ) which came to be known as the
labarum. The etymology of the word before Constantine's usage of it is unclear. According to
Lactantius (
On the Deaths of Persecutors, chapter 44), Constantine had dreamed of this emblem and a voice saying "In this sign you will conquer" (Latin:
In hoc signo vinces). On waking he ordered his soldiers to put the emblem on their shields; that day they fought and won the
Battle of the Milvian Bridge against the forces of
Maxentius.
The labarum is sometimes referred to as the monogram of Christ.