One day before the Battle of Milvian Bridge, on October 27, 312, Constantine had a vision assuring him of victory in the name of the Christian God. Constantine controlled the Eastern Roman Empire. There was a "tetrarchy" of rulers: Constantine, Maxentius, Maximian and Galerius.
The Vision of the Cross by Raphael, 1520-24.
Maxentius revolted in Rome and Constantine went to confront him. On the night before the battle, Constantine had a dream and came across with the letters: In hoc signo vinces (In this rule). The next day Constantine made a banner with the cross and did put crosses on all the shields of his soldiers and happened to defeat Maxentius. So it is said that Constantine became a Christian. It was the first Christian emperor and his death was sanctified.
Constantine Sees a Vision of the Cross in the Sky Before the Battle at Milvian Bridge.
That story has not truthful sources, however. Even it has been said that a higher clergy friend of Constantine insisted him on not persecuting Christians because they were majority. If accepted, he would win their favor. Constantine found there an opportunity to govern with the support of the Christian people.
Constantine's Vision of the Christian Cross before the Battle of the Milvian Bridge Lámina giclée.
Later, Constantine met with Lycinus, who controlled the Balkans, in Milan. They agreed that the policy should continue to be benevolent with ragged Christians. When Constantine was dying, his bishop friend went to the balcony to report that Constantine had converted into Christianity while on his deathbed… and the news spread by generating new stratification to Christianity and Constantine was eventually sanctified.
Only Constantine and his soldiers saw the VISION IN THE SKY.