A vision in the midday sky appeared to Emperor Constantine (274 - 306 A.D.) in the form of a flaming cross, consisting of a long golden spear with a cross bar, and above it a golden wreath with precious stones. The letters X and P were also a part of this vision, representing the name of Christ ( the greek lettering Χριστος representing Christos or Christ, and P representing a monogram of Christ).
At the same time, Constantine also heard the words "In Hoc Signo Vinces" (translated as "In this sign you shall conquer"). Constantine believed this vision to be a sign from God, and took the shape of the symbol and made it as an emblem for his military. The emblem became later referred to as "Labarum" (the meaning of this word is unclear, though it probably has Gaulish/Celtic roots).
Constantine placed the emblem on his soldiers' shields and at the top of his standard at the Battle Of Milvain Bridge near Rome, in A.D. 312, which he won. After this victory, Constantine became emperor of the West. He openly supported Christianity from that point forward, and was know as the thirteenth apostle.
The cross symbol of Constantine's vision replaced the eagle as the military standard and emblem for the Roman army, and was later replaced by a simplified cross. Called the Chi Rho cross, or Constantine's Cross, the cross is seen in several variations, one of which is the letter P with a horizontal line crossing the P's tail to form the shape of a cross. In another variation, a diagonal X intersects the tail of the letter P.