The Chi-Rho emblem can be viewed as the first Christian Cross.
As a pre-Christian symbol, the Chi-rho signified good fortune. The Chi ro became an important Christian symbol when adopted by the Roman Emperor Constantine, representing the first two letters in the name of Christ- the Chi, or ‘ch,’ and Rho, or ‘r.’ According to Church Father Eusebius, on the eve of the Battle of the Milvan Bridge, the Emperor saw the emblem in a dream, with the inscription, “By this sign, you shall conquer.” According to the story, the battle was won. In return for the victory, Constantine legalized the religion and erected Christian churches.
There is some speculation that the ‘sign’ witnessed by Constantine was an occurrence of the sun dogs phenomena.
Unfortunately, this story is very unlikely, as Constantine’s conversion occured on his deathbed, if at all. In any case, the symbol was the standard of the Emperor’s army, prominently displayed on the Emperor’s labarum, or battle standard.
Before it became the monogram of Christ, the chi rho was the monogram of Chronos (whose name also begins with a Chi-rho), the god of time, and an emblem of several solar deities.
The Chi-ro is also the origin of the tradition of abbreviating “Christ” in “christian” or “Christmas” to “X.” The small letters in the image are the alpha and omega.
In Hebrew, Chi-Rho equates to Tav-Resh. The chi rho was used in hermetic alchemical texts to denote time.