History of Christianity
The first to legalize Christianity was the Armenian king Trdat the Third, who announced it the official religion in Armenia in the year 301. Galerius issued an edict permitting the practice of the Christian religion in April of 311. In 313 Constantine I and Licinius announced toleration of Christianity in the Edict of Milan.  Constantine became the first Christian emperor; he had learnt about Christianity from his mother, Helena.