The Chi Rho is one of the earliest cruciform symbols used by Christians. It is formed by superimposing the first two letters of the word "Christ" in Greek.
Although not technically a cross, the Chi Rho invokes the crucifixion of Jesus as well as symbolizing his status as the Christ. On either side is the Alpha and Omega, which symbolizes Christ's Lordship over all time.
This beautifully carved monogram, surrounding by the wreath of victory, a symbol of Christ's resurrection, is from an early Christian sarcophagus in the Vatican Museum.