XP, (chi-rho, the first letters in the Greek word for Christ; a Christogram). He was instructed in the vision, "Under this sign you will be victorious". He placed the symbol on his helmet and on the shields of his soldiers. During the battle the horse of the opposing general was spooked and threw Maxentius in full armor into the river, where he drowned. So Constantine was indeed victorious at the battle of Milvan Bridge on October 28, 312.
Confident that his victory over Maxentius was achieved by divine intervention of the Christian God, he issued the Edict of Milan in 313 granting complete religious toleration throughout the Empire. He became a Christian himself and eventually proclaimed Christianity as the official religion of his empire. Thus the major persecutions ended. This was also the start of nominal Christianity as the church was flooded with non-believers.
The back of this coin shows two soldiers beside a standard with the XP. Though its use on coins was uncommon for Constantine, subsequent emperors increasing used it. Some also used the message of the vision on their coins.
[SR 3787]