Christianity was initially adopted by Roman Matrons as a way of avoiding infanticide. Newborns were sometimes sacrificed to the Gods and if a Roman woman became a Christian she could cite her religion as a reason to spare her child.
Constantine the Great, Emperor of Rome heavily favored Christianity, in part due to his mother Helena's influence, though the entirety of his motives are unknown. It was referred to as the Triumph of the Church and set a precendent for a Christian Monarch which prevailed for centuries.
As a result, Crucifixion was made illegal (but was replaced by hanging) and Sunday was declared to be a day of rest. During his reign Constantine also began to tear down Roman temples.