Constantine

Constantine had a vision in which a flaming cross appeared in the sky with an inscription over it reading in Latin, "In Hoc Signo Vinces" - In This Sign Conquer! He dedicated the battle to the Christian God (whose sign he believed he was shown) and upon victory became supreme ruler in the Western Roman Empire. Christianity had finally gained favor in high places.
In 313 A.D., Constantine issues the Edict of Milan which gives Christians the freedom to worship openly throughout the empire, putting the growing new religion on an equal plane with the popular pagan cults and practices that thrived there.