Constantine the Great, Emperor of Rome, dies at the age of 47. Constantine gave Christianity social and political legitimacy in the Roman Empire by granting it toleration and by making it his own personal religion.
When Constantine ascended the throne the Roman Empire was fragmented and in disarray. His chief goal was thus to create and maintain unity - political, economic, and religious.
Christianity fit Constantine's need for a basis of religious unity quite well. Christians were a minority in the empire but they were a well-organized minority which no one had tried to use in a political alliance.
Because of Constantine's desire for unity, he ruthlessly enforced his own brand of orthodoxy among Christians. There as no toleration of weakness through division, strife, or disagreement. Constantine appropriated this authority for himself by declaring that he was a "bishop, ordained by God."
First Constantine eliminated the external challenges posed by paganism, destroying their temples and books. Then he dealt with internal challenges by ordering that "unorthodox" Christian groups be eliminated.
Very quickly, theological disagreements which had been a part of the Christian experience became "unchristian." In this way, a choice ("heresy") to be religiously different became defined as treason, a political crime.