Constantine's reason for championing Christianity was purely in order to gain total control of the Roman world.
Pagan thug makes Christian Emperor
It was October, 312 AD Constantine and his troops marched toward Rome to do battle with his opponent, another would-be emperor, the tyrannical Maxentius.
As Constantine stopped to consider his battle plans, he realized he needed greater power than just military force. Maxentius was relying on magical enchantments. But how should Constantine pray? His father was a monotheist, believing there was only one God over all. Constantine saw that the pagan gods failed to protect their worshipers. So he sought his father's God in prayer, pleading for him to tell him who he was and to stretch forth his hand to help him. As he prayed (it was a little after noon), Constantine had an absorbing vision. He saw the sign of the cross emblazoned across the sky and the words In hoc signo vinces, "In this sign you will win."
Constantine accepted the vision. He adopted the sign. He had the cross inscribed on his soldiers' armor. He went into battle. Even though his forces were outnumbered, he won.