One day in 312 A.D. Constantine decided to move against his brother-in-law Maxentius who had become a tyrant in Rome. Considering that the gods had not helped others who had moved against Maxentius, he decided to pray for help to the true God. The historian Eusebius tells us that Constantine and his army saw a cross of light above the lowering sun and an inscription which read "In Hoc Signo Vinces" (in this sign, conquer). That same night Constantine in a dream saw Christ with the same sign who told him to make an image of the sign as safety against his enemies. The sign which Constantine made consisted of a spear with a transverse and on top of it a wreath of gold with the first and last letter of the name Christ (chi-rho), a banner hung from the transverse with the portrait of the emperor and his children. "The opposing forces met near the bridge over the Tiber called the Milvian Bridge, and here Maxentius' troops suffered a complete defeat, the tyrant himself losing his life in the Tiber (28 October, 312)." (Advent Catholic Digital Encyclopedia) Now Constantine decided to make Christianity the favored official religion of the Empire.
Edict of Milan 313 A.D.
by Constantine Augustus and Licinius Augustus
"Therefore, your Worship should know that it has pleased us to remove all conditions whatsoever, which were in the rescripts formerly given to you officially, concerning the Christians and now any one of these who wishes to observe Christian religion may do so freely and openly, without molestation."
Saint HelenaConstantine was instrumental in bringing about the conversion of his mother Helen in 312 A.D. She became very devoted to Christ and did much to spread Christianity. Helena was very generous to the poor and given to building Churches. Worthy of note is that she supervised the building the Church in Bethlehem over the Grotto of the Nativity and another church on the Mount of the Ascension.
She is usually depicted with a Cross because on her pilgrimage to Jerusalem, it is said, she discovered the Cross of Christ on September 14, 320 A.D. Upon arriving to the Holy Land, she discovered that the site of the Holy Sepulcher was covered over by a temple built by Hadrian to Venus. She ordered the temple torn down and in the process discovered in the tomb of Christ three wooden crosses. Which one was the Cross of Christ? A sick person laid upon it was instantly cured and in this way she recognized the true Cross. On this site she built the Church of the Holy Sepulcher. The Imperial Order of Saint Helena traces itself back to the Emperor Constantine who established it to commemorate the finding of the True Cross by his mother Helena. If we accept this claim, it would give historical value to this event which many regard a legend.
St. Helena died at the age of 80 c. 330 A.D. and Constantine was with her when she died. Her body was brought to Constantinople and laid to rest in the imperial vault of the church of the Apostles.
. to Augustine City of God