Chi Rho

In pre-Christian times, the Chi-Rho symbol was also used to mark a particularly valuable or relevant passage in the margin of a page, abbreviating chrēston (good).

According to Lactantius,[6] a Latin historian of North African origins saved from poverty by the Emperor Constantine the Great (r. 306–337), who made him tutor to his son Crispus, Constantine had dreamt of being ordered to put a "heavenly divine symbol" (Latin: coeleste signum dei) on the shields of his soldiers.

That night, Christ appeared to the Roman emperor in a dream and told him to make a replica of the sign he had seen in the sky, which would be a sure defence in battle.

Eusebius wrote in the Vita that Constantine himself had told him this story "and confirmed it with oaths" late in life "when I was deemed worthy of his acquaintance and company."

Eusebius also left a description of the labarum, the military standard which incorporated the Chi-Rho sign, used by Emperor Constantine in his later wars against Licinius.

[14][15] In Insular Gospel books, the beginning of Matthew 1:18, at the end of his account of the genealogy of Christ and introducing his account of the life, so representing the moment of the Incarnation of Christ, was usually marked with a heavily decorated page, where the letters of the first word "Christi" are abbreviated and written in Greek as "XPI", and often almost submerged by decoration.

[17] The "X" was regarded as the crux decussata, a symbol of the cross; this idea is found in the works of Isidore of Seville and other patristic and Early Medieval writers.

In some other works like the Carolingian Godescalc Evangelistary, "XPS" in sequential letters, representing "Christus" is given a prominent place.