Catacombs of Domitilla Chi Rho Recreation (about 3 1/2 iches by 2 1/4 inches, resin cast). The Chi Rho is formed by superimposing the first two (capital) letters chi and rho (ΧΡ) of the Greek word "ΧΡΙΣΤΟΣ" =Christ in such a way to produce the monogram. Widespread in ancient Christianity, it was the symbol used by the Roman emperor Constantine I as vexillum (named Labarum)Wikipedia. Also used since the earliest Christianity is the first and the last letters of the Greek alphabet, alpha (α or Α) and omega (ω or Ω), derives from the statement said by Jesus himself "I am the alpha and the omega, the beginning and the end" (Revelation 22:13, also 1:8 and 21:6).